Short definition
A sitemap is a machine-readable list of every important URL on a website, submitted to search engines to ensure complete and up-to-date indexing.
In depth
Sitemaps tell crawlers which URLs exist, when they were last modified, how often they change, and their relative priority. Large or fast-moving sites split into a sitemap index that references multiple sub-sitemaps (pages, images, news, video). Sitemaps do not guarantee indexation but materially speed it up. They are referenced from robots.txt and submitted directly to Google Search Console and Bing Webmaster Tools.
Example
A new content hub publishes 60 articles, glossary terms, and comparison pages. A sub-sitemap surfaces them with lastmod dates; Google indexes 90% within three weeks.