Glossary
The SEO glossary, in plain English
164 terms from technical SEO, content, performance and AI search, each with an example and what it means for a WordPress site.
# 11
- .htaccessThe .htaccess file is a plain-text configuration file read by Apache-compatible web servers on each request, used to set redirects, URL rewriting, headers and access rules for the directory it sits in.
- 301 redirectA 301 redirect is an HTTP response telling browsers and search engines that a URL has moved permanently to a new address, so links and rankings should transfer to the new URL.
- 302 redirectA 302 redirect is an HTTP response that sends visitors to a different URL temporarily, signaling to search engines that the original URL should stay indexed.
- 304 Not Modified304 Not Modified is the HTTP status a server returns to a conditional request when the resource has not changed, telling the client to reuse the copy it already holds instead of downloading it again.
- 307 redirectA 307 Temporary Redirect is an HTTP status telling the client that a resource is temporarily at another URL and that it must repeat the request there with the same method and body.
- 308 redirectA 308 Permanent Redirect is an HTTP status that says a resource has moved for good and that the client must repeat the request at the new URL with the same method and body.
- 403 Forbidden403 Forbidden is the HTTP status a server returns when it understands a request but refuses to fulfil it, typically because of permissions, a firewall rule or an access restriction.
- 404 errorA 404 error is the HTTP Not Found status a server returns when a requested URL does not exist. Some 404s are normal; broken links to them and lost backlinks are what cost you.
- 410 Gone410 Gone is an HTTP status code that tells browsers and search engines a URL has been removed on purpose and will not come back, a more explicit version of a 404.
- 429 Too Many Requests429 Too Many Requests is the HTTP status a server or firewall returns when a client has exceeded a rate limit, asking it to slow down, optionally with a Retry-After header saying when to try again.
- 503 Service Unavailable503 Service Unavailable is an HTTP status code meaning the server cannot handle the request right now, usually due to maintenance or overload, and should be tried again later.
A 11
- Above the foldAbove the fold is the part of a web page visible without scrolling. On the web it changes with screen size, and it matters for loading speed, layout and what readers see first.
- AI citationAn AI citation is a reference, usually a clickable link, that an AI assistant or AI search feature attaches to its answer to show which web page a statement was drawn from.
- AI crawlerAn AI crawler is an automated bot operated by an AI company that fetches web pages, either to collect training data for language models, to build a search index for AI answers, or to read a page on a user's request.
- AI ModeAI Mode is Google's conversational search experience, launched in the US in 2025, that answers complex questions with an AI response, accepts follow-ups, and links to web sources.
- AI OverviewsAI Overviews are AI-generated summaries that Google shows at the top of some search results, answering the query directly and linking to the web pages used as sources.
- Alt textAlt text is the written description of an image in its HTML alt attribute. Screen readers read it aloud, browsers show it when the image fails to load, and search engines use it.
- Anchor textAnchor text is the visible, clickable text of a hyperlink. Search engines use it as a description of the page being linked to.
- Answer engine optimization (AEO)Answer engine optimization (AEO) is the practice of structuring content so AI assistants such as ChatGPT, Perplexity, and Google's AI features can find, understand, and cite it in their answers.
- Attachment pageAn attachment page is the public page WordPress creates for every file uploaded to the Media Library, showing that one image or file inside the theme's layout, usually with nothing but its title.
- Author archiveAn author archive is the listing page WordPress generates for each user who has published posts, at an address such as /author/jane/, showing that person's posts newest first.
- Average positionAverage position is a Google Search Console metric giving the mean position of your site's topmost link in search results, averaged over all the impressions for a query, page or the whole property.
B 5
- BacklinksBacklinks are links from other websites pointing to pages on your site. Search engines treat them as signals of trust and relevance, and they remain an important ranking factor.
- Bing Webmaster ToolsBing Webmaster Tools is Microsoft's free console for site owners. It reports how Bing crawls, indexes and ranks your site and, since February 2026, how often Copilot and Bing AI answers cite your pages.
- BingbotBingbot is Microsoft's web crawler. It discovers and fetches pages for the Bing search index, which also supplies results to Microsoft Copilot and to other search products that license Bing's data.
- Bounce rateBounce rate in Google Analytics 4 is the percentage of sessions that were not engaged, meaning they lasted under 10 seconds, had no key event, and included only one page or screen view.
- BreadcrumbsBreadcrumbs are a secondary navigation trail, such as Home > Guides > Coffee, that shows where a page sits in a site's hierarchy and links back up to each parent level.
C 15
- Canonical URLA canonical URL is the preferred address of a page, declared with a rel="canonical" link tag, that tells search engines which of several duplicate URLs to index and credit.
- Category baseThe category base is the fixed prefix, /category/ by default, that WordPress places before a category's slug in the URL of its archive page, as in example.com/category/news/.
- CDNA CDN (content delivery network) is a network of servers in many locations that stores copies of a site's files and serves each visitor from a nearby location to cut load time.
- Character encodingCharacter encoding is the system that maps a page's bytes to readable characters. Declaring it, almost always as UTF-8, lets browsers and crawlers display text correctly.
- Click-through rate (CTR)Click-through rate (CTR) is the percentage of people who click a result after seeing it, calculated as clicks divided by impressions. In SEO it is measured per page and query.
- Content decayContent decay is the gradual decline in search traffic and rankings of a page that once performed well, usually because it aged, competitors improved, or intent shifted.
- Core updateA core update is a significant, broad change Google makes to its core ranking systems, several times a year. It isn't aimed at particular sites, and Google announces the notable ones.
- Core Web VitalsCore Web Vitals are three Google metrics for real-user experience: Largest Contentful Paint (loading), Interaction to Next Paint (responsiveness), and Cumulative Layout Shift (visual stability).
- Crawl budgetCrawl budget is the number of URLs a search engine is able and willing to crawl on a site over a period of time, set by server capacity and how much the engine wants the content.
- Crawl depthCrawl depth, also called click depth, is the number of clicks it takes to reach a page from the home page by following internal links. Pages buried many clicks deep are usually found later and revisited less often.
- Crawl stats reportThe Crawl stats report is a Google Search Console report showing Google's crawling activity on a site over the last 90 days: request counts, download size, response times, response codes and host availability.
- Crawl-delayCrawl-delay is an unofficial robots.txt directive that asks a crawler to wait a set number of seconds between requests. Bingbot and some other crawlers honour it, while Googlebot ignores it completely.
- CrawlingCrawling is the process by which search engine bots such as Googlebot discover URLs and download their content, mainly by following links and reading sitemaps.
- Cumulative Layout Shift (CLS)Cumulative Layout Shift (CLS) is a Core Web Vital that scores how much visible content moves unexpectedly while a page is open, such as text jumping when an image loads.
- Custom post typeA custom post type is a kind of content registered in WordPress in addition to the built-in posts and pages, such as products, events or recipes, with its own URLs, archive, editor fields and templates.
D 6
- DisavowDisavowing links means submitting a file through Google Search Console that asks Google to ignore specific backlinks or entire domains when assessing your site.
- DoctypeA doctype is the <!DOCTYPE html> declaration at the very top of an HTML document that tells browsers to render the page in standards mode rather than legacy quirks mode.
- Doorway pagesDoorway pages are pages or sites created to rank for many similar searches that funnel visitors to one destination. Google's spam policies call the practice doorway abuse.
- Duplicate contentDuplicate content is the same or nearly the same content reachable at more than one URL, on one site or across sites. Search engines index one version and filter out the rest.
- Duplicate, Google chose different canonical than user"Duplicate, Google chose different canonical than user" is a Search Console status for a page whose canonical tag names one URL while Google has judged a different URL to be the representative version and indexed that instead.
- Dynamic renderingDynamic rendering means detecting crawlers by user agent and sending them a prerendered, static HTML version of a JavaScript-heavy page, while human visitors receive the normal client-rendered app.
E 3
- E-E-A-TE-E-A-T stands for Experience, Expertise, Authoritativeness, and Trustworthiness, the framework Google's human quality raters use to judge content. It is not a direct ranking factor.
- EntityIn SEO, an entity is a distinct, identifiable thing, such as a person, business, place, product or concept, that search engines can recognize regardless of the exact words used to name it.
- Evergreen contentEvergreen content is material that stays useful and accurate for years, such as how-tos, definitions and reference guides, as opposed to news or trend pieces that go stale quickly.
F 3
- Faceted navigationFaceted navigation is a set of filters on a listing page, such as color, size, brand, and price, that narrows results and often creates a new URL for every combination.
- FaviconA favicon is the small icon that identifies a website in browser tabs, bookmarks, and mobile search results, declared with a link rel="icon" tag in the page head.
- Featured snippetA featured snippet is a highlighted box in Google results that quotes a short answer from one web page, as a paragraph, list, or table, with a link to the source.
G 6
- Generative engine optimizationGenerative engine optimization (GEO) is the practice of making content more likely to be used and cited by AI systems that generate answers, such as ChatGPT, Perplexity, and Google AI Mode.
- Google Business ProfileA Google Business Profile is a free listing, formerly called Google My Business, that shows a business's details, hours, photos, and reviews in Google Maps and local search.
- Google DiscoverGoogle Discover is a personalized feed of articles and videos in the Google app and on Google's mobile homepage, shown based on a user's interests rather than a typed query.
- Google Search ConsoleGoogle Search Console is a free service from Google that shows how a site appears in Google Search, including queries, clicks, indexing status, and technical problems.
- Google-ExtendedGoogle-Extended is a robots.txt product token, not a separate crawler, that lets site owners control whether content Google crawls may be used to train Gemini models and to ground Gemini's answers.
- GooglebotGooglebot is the web crawler Google Search uses to fetch pages and their resources. It runs as a smartphone crawler and a desktop crawler, and both obey the same robots.txt token.
H 6
- H1 tagAn H1 tag is the top-level HTML heading element (<h1>) on a page, normally used for the main visible title that tells readers and search engines what the page is about.
- HreflangHreflang is an attribute on link tags that tells search engines which language and regional versions of a page exist, so each searcher is shown the version meant for them.
- HSTSHSTS, HTTP Strict Transport Security, is a response header that instructs browsers to connect to a domain over HTTPS only for a set period, converting any HTTP request before it leaves the browser.
- HTML sitemapAn HTML sitemap is an ordinary web page that links to a site's important pages, organized for people to browse. It is separate from an XML sitemap, which is a file written for crawlers.
- HTTP status codesHTTP status codes are three-digit numbers a server returns with every response, telling browsers and crawlers whether the request succeeded, was redirected, or failed.
- HTTPSHTTPS is the encrypted version of HTTP. It uses TLS to protect data sent between a browser and a website, and Google uses it as a lightweight ranking signal.
I 8
- Image SEOImage SEO is the practice of making images discoverable and understandable to search engines, through alt text, file names, formats, sizing, and markup, while keeping pages fast.
- Image sitemapAn image sitemap is an XML sitemap, or an extension of one, that lists the images on each page with image:image tags so search engines can discover them for image search.
- ImpressionsIn Google Search Console, an impression is counted each time a link to your site appears in search results that a user loads, whether or not the user scrolls far enough to see it or clicks it.
- Index bloatIndex bloat is when a search engine has indexed far more of a site's URLs than the site has pages worth ranking, usually thin archives, parameter URLs, and duplicates.
- IndexingIndexing is the process in which a search engine analyzes a crawled page and stores it in its index, the database it draws on to answer searches. Only indexed pages can rank.
- IndexNowIndexNow is an open protocol that lets a website notify participating search engines, such as Bing and Yandex, immediately when a URL is added, updated, or deleted.
- Interaction to Next Paint (INP)Interaction to Next Paint (INP) is a Core Web Vital that measures how quickly a page visibly responds to clicks, taps, and key presses across the whole visit.
- Internal linkingInternal linking is the practice of linking from one page on a site to another page on the same site, helping visitors navigate and search engines discover and understand pages.
J 2
- JavaScript SEOJavaScript SEO is the practice of making sure pages that rely on JavaScript to build content, links, or metadata can still be crawled, rendered, and indexed by search engines.
- JSON-LDJSON-LD (JavaScript Object Notation for Linked Data) is a W3C format for expressing structured data as JSON, and the format Google recommends for schema.org markup.
K 5
- Keyword cannibalizationKeyword cannibalization is when two or more pages on the same site target the same search query and intent, splitting signals so that none ranks as well as one strong page would.
- Keyword researchKeyword research is the process of finding the words and phrases people type into search engines about a topic, then judging which are worth targeting with a page.
- Keyword stuffingKeyword stuffing is the practice of filling a page with keywords or numbers in an attempt to manipulate search rankings, often in lists, repeated phrases or hidden text that read unnaturally.
- Knowledge GraphThe Knowledge Graph is Google's database of entities, such as people, places, organizations and things, and the facts and relationships between them. It feeds knowledge panels and other direct answers.
- Knowledge panelA knowledge panel is an information box in Google's results about a person, organization, place, or thing, built from Google's Knowledge Graph of known entities.
L 8
- Largest Contentful Paint (LCP)Largest Contentful Paint (LCP) is a Core Web Vital that measures the time from when a page starts loading until its largest visible image or text block is rendered.
- lastmodlastmod is the optional XML sitemap element that gives the date, and optionally the time, a URL's content was last significantly modified, helping crawlers decide which pages to fetch again.
- Lazy loadingLazy loading is a technique that defers loading off-screen images, iframes, and other resources until the user scrolls near them, reducing initial page weight and load time.
- Link equityLink equity is the SEO shorthand for the ranking value one page passes to another through a link. Google's name for its best-known version is PageRank, which it says is still part of its core ranking systems.
- llms.txtllms.txt is a proposed convention: a Markdown file at a site's root (/llms.txt) that summarizes the site and links its most important pages for large language models to read.
- Local SEOLocal SEO is the work of making a business visible in searches with local intent, such as "dentist near me", across Google Maps, the local pack, and regular results.
- Log file analysisLog file analysis means reading your web server's access logs to see exactly which URLs search engine and AI crawlers requested, when, and which status code your server returned.
- Long-tail keywordsLong-tail keywords are specific, usually longer search phrases with low individual search volume that are easier to rank for and often match a clear need.
M 8
- max-image-previewmax-image-preview is a robots meta directive that sets the largest image preview a search engine may show for a page, with the values none, standard and large.
- max-snippetmax-snippet is a robots meta directive that limits the text snippet a search engine may show for a page to a set number of characters.
- Meta descriptionA meta description is an HTML meta tag that summarizes a page's content. Search engines often, but not always, show it as the snippet text below the result title.
- Meta refreshA meta refresh is an HTML meta tag that tells the browser to reload the page or load a different URL after a given number of seconds, working as a client-side redirect.
- Meta robots tagThe meta robots tag is an HTML element in a page's head that gives search engines page-level instructions such as noindex, nofollow, noarchive, and snippet limits.
- MicrodataMicrodata is an HTML specification for embedding structured data directly in a page's visible markup, using the attributes itemscope, itemtype and itemprop to label elements with schema.org types and properties.
- Mixed contentMixed content occurs when a page served over HTTPS loads some of its resources, such as images, scripts, stylesheets or iframes, over unencrypted HTTP, which browsers block or flag as not fully secure.
- Mobile-first indexingMobile-first indexing means Google predominantly crawls and indexes the mobile version of a page and uses that version's content for ranking, including desktop results.
N 6
- NAP consistencyNAP consistency means a business's name, address, and phone number are written the same way on its website, Google Business Profile, directories, and social profiles.
- News sitemapA news sitemap is an XML sitemap with extra news tags that lists a publisher's articles from the last two days, giving Google the title and publication date of each so new stories are discovered quickly.
- noarchivenoarchive is a robots directive that asks search engines not to store or show a cached copy of a page. Google no longer acts on it since removing its cache link, while Bing still reads it.
- NofollowNofollow is a rel attribute value on a link (rel="nofollow") that asks search engines not to pass ranking credit through it. Since 2019 Google treats it as a hint rather than a strict rule.
- NoindexNoindex is a robots directive, set in a meta tag or HTTP header, that tells search engines they may crawl a page but must not include it in search results.
- nosnippetnosnippet is a robots directive that tells search engines not to show a text snippet or video preview for a page in results. The related data-nosnippet attribute excludes a single passage instead.
O 2
- Open GraphOpen Graph is a protocol of meta tags (og:title, og:description, og:image and others) that controls the title, description, and image shown when a link is shared on social platforms.
- Orphan pageAn orphan page is a page on a website that no other page on the same site links to, so visitors cannot navigate to it and crawlers can find it only through sitemaps or external links.
P 8
- Page indexing reportThe Page indexing report is the Google Search Console report that counts a site's indexed and not-indexed URLs and groups the not-indexed ones by the reason Google gives for each.
- Page speedPage speed is how quickly a web page loads, displays its content, and responds to input, measured with metrics such as TTFB, LCP, and INP rather than one single number.
- PageRankPageRank is a Google algorithm that estimates the importance of a web page from the number and importance of the pages that link to it, treating each link as a weighted vote.
- PaginationPagination is splitting a long list of items, such as blog posts or products, across a numbered series of pages (/page/2/, /page/3/) linked to one another.
- People Also AskPeople Also Ask (PAA) is a Google results feature showing related questions that expand to reveal a short answer from a web page with a link to the source.
- PermalinkA permalink is the permanent URL of a specific post, page, or archive. In WordPress the term also refers to the site-wide structure that decides how those URLs are built.
- Pillar pageA pillar page is the central page on a subject: it covers the topic broadly, targets the widest search for it, and receives internal links from the more specific pages around it.
- Primary categoryA primary category is the single category chosen to represent a post that belongs to several, used in the post's URL, breadcrumb trail and structured data.
Q 1
R 10
- Redirect chainA redirect chain is a sequence of two or more redirects between the first requested URL and the final page, such as A to B to C, which slows loading and crawling.
- Redirect loopA redirect loop is a chain of redirects that eventually points back to a URL already visited, so the browser or crawler circles until it gives up and no page is ever returned.
- Referring domainsReferring domains are the unique websites that have at least one link pointing to your site. One referring domain may account for a single backlink or for thousands of them.
- rel="noopener"rel="noopener" is a link attribute that stops a page opened in a new tab from controlling the page that opened it. It is a security setting and has no effect on rankings or link value.
- Removals toolThe Removals tool is a Google Search Console feature that temporarily hides URLs on your own site from Google Search results for about six months, or clears their stored snippet until the next crawl.
- Retrieval-augmented generation (RAG)Retrieval-augmented generation, or RAG, is a technique in which an AI system first retrieves relevant documents from a search index or database and then has a language model write its answer using that retrieved text.
- Rich resultsRich results are search listings that show more than a title, URL, and snippet, such as star ratings, prices, images, or event dates, usually generated from structured data.
- Rich Results TestThe Rich Results Test is a free Google tool that fetches a URL or a pasted code snippet, reads its structured data, and reports which rich result types the page is eligible for, with any errors and warnings.
- robots.txtrobots.txt is a plain-text file at the root of a domain that tells web crawlers which URL paths they are allowed to request. It controls crawling, not indexing.
- RSS feedAn RSS feed is a machine-readable XML file that lists a site's most recent posts with their titles, links, dates and text, so feed readers, apps and other services can pick up new content automatically.
S 18
- sameAssameAs is a schema.org property holding URLs of other pages that unambiguously identify the same thing, such as an organization's official social profiles or a person's entry in a reference database.
- Scaled content abuseScaled content abuse is a Google spam policy, introduced in March 2024, against generating many pages mainly to manipulate rankings rather than help people, however they are made.
- Schema markupSchema markup is structured data that uses the schema.org vocabulary to describe what a page contains, such as an article, product, or event, in a form machines can read directly.
- Search engine visibilitySearch engine visibility is a WordPress setting under Settings → Reading. Ticking "Discourage search engines from indexing this site" adds a noindex, nofollow robots meta tag to every page.
- Search intentSearch intent is the underlying goal a person has when typing a query, such as learning something, reaching a specific site, comparing options, or making a purchase.
- Search operatorsSearch operators are special commands, such as site: and filetype:, that narrow a Google search. Site owners use them for quick checks, but they aren't a reliable count of indexed pages.
- Search Quality Rater GuidelinesThe Search Quality Rater Guidelines are Google's public handbook for the people it hires to rate search results. Their ratings help Google test ranking changes; they don't rank individual pages.
- Self-referencing canonicalA self-referencing canonical is a rel="canonical" link whose href is the clean URL of the very page it sits on, telling search engines that this address is the preferred one for the content.
- SERPA SERP (search engine results page) is the page a search engine shows in response to a query, containing organic results, ads, and features such as AI Overviews and local packs.
- Server-side renderingServer-side rendering (SSR) is generating a page's complete HTML on the server for each request, so browsers and crawlers receive the content without having to run JavaScript first.
- Site nameA site name is the name of a website that Google displays above the URL and title link in a search result. Google generates it automatically from the homepage's structured data, title and other signals.
- Site verificationSite verification is how you prove to a webmaster console such as Google Search Console or Bing Webmaster Tools that you control a website, by placing a code the service issues where only the site's owner could put it.
- SitelinksSitelinks are additional links to pages within a site that Google shows beneath that site's main search result, mostly for brand and navigational searches, chosen automatically by Google's algorithms.
- Sitemap indexA sitemap index is an XML file that lists the locations of other sitemap files, letting a site split its URLs across many smaller sitemaps and submit them with one URL.
- Soft 404A soft 404 is a URL that returns a success status (200 OK) but shows content Google judges to be an error, empty, or missing page, so Google treats it as not found.
- Spam updateA spam update is a notable improvement Google makes to its automated spam detection systems, such as SpamBrain. Sites that violate Google's spam policies can rank lower or disappear afterward.
- Sponsored and UGC linksSponsored and UGC links are links marked rel="sponsored" (paid or affiliate) or rel="ugc" (user-generated content), attributes Google has recognized as hints since 2019.
- Staging siteA staging site is a private copy of your website where you test updates, plugins and redesigns before applying them to the live site. It should be blocked from crawling and indexing.
T 9
- Table of contentsA table of contents is a list of links, usually near the top of a long page, where each link jumps to one section of that same page. The links are often called jump links or anchor links.
- TaxonomyA taxonomy in WordPress is a system for grouping content. Categories and tags are the two built-in ones, and each individual group within a taxonomy, such as "News", is called a term.
- Thin contentThin content is a page that offers little or no original value to a visitor, such as near-empty pages, doorway pages, scraped or spun text, or affiliate pages with nothing added.
- Time to First Byte (TTFB)Time to First Byte (TTFB) is the time between a browser requesting a page and receiving the first byte of the response, covering redirects, DNS, connection, and server work.
- Title tagA title tag is the HTML <title> element that names a page. Search engines usually show it as the clickable headline of a result, and browsers show it in the tab.
- Topic clustersA topic cluster is a group of pages on one subject: a broad pillar page that links to detailed pages on each subtopic, which link back to the pillar and to each other.
- Topical authorityTopical authority is the SEO idea that a site which covers a subject thoroughly and well becomes a more trusted source for queries about that subject.
- Trailing slashA trailing slash is the forward slash at the end of a URL path. Apart from the bare hostname, a URL with it and the same URL without it are two different addresses to a search engine.
- Twitter cardA Twitter card is the preview X (formerly Twitter) shows for a shared link, controlled by twitter: meta tags such as twitter:card, twitter:title, and twitter:image.
U 6
- URL fragmentA URL fragment is the part of a URL after the # sign. It identifies a position or element within the page, is handled entirely by the browser, and is never sent to the server.
- URL Inspection toolThe URL Inspection tool is a Google Search Console feature that reports the index status of a single URL on your site, lets you test the live page, and lets you request indexing.
- URL parametersURL parameters are the key=value pairs after the question mark in a URL, used for tracking, sorting, filtering and search. Each different combination can look like a separate page to a crawler.
- URL slugA URL slug is the final, human-readable part of a URL path that identifies a specific page, such as pour-over-guide in example.com/blog/pour-over-guide/.
- User agentA user agent is the identifying string a browser, crawler or other client sends in the User-Agent HTTP header with each request. Robots.txt rules and server logs use it to tell one crawler from another.
- UTM parametersUTM parameters are query string tags, such as utm_source and utm_medium, added to a link's URL so that analytics tools can attribute the resulting visit to a specific source, medium and campaign.
V 2
- Video sitemapA video sitemap is an XML sitemap with video tags that tells search engines which pages contain a video and supplies each video's thumbnail, title, description and file or player URL.
- Viewport meta tagThe viewport meta tag is an HTML tag that tells mobile browsers to match the page width to the device's screen, which is required for a responsive layout to display properly.
W 1
X 3
- x-defaultx-default is a reserved hreflang value that marks the page search engines should show to users whose language and region match none of the other versions listed, typically a language selector or a global default page.
- X-Robots-TagThe X-Robots-Tag is an HTTP response header that carries robots directives such as noindex or nosnippet, letting you control indexing of any file type, not just HTML pages.
- XML sitemapAn XML sitemap is a machine-readable file that lists the URLs on a site you want search engines to crawl and index, usually with the date each page last changed.