Sitemap.xml is an XML file at /sitemap.xml (or a path declared in robots.txt) that lists every indexable URL on a site, optionally with lastmod, changefreq, and priority hints. Recognized by Google, Bing, and most major search engines as the canonical content-discovery manifest.
For SEO, sitemap.xml is most valuable for large sites (>1000 URLs) where Google's natural crawl might miss content, and for sites with new pages that need fast indexation. For small sites with strong internal linking, the sitemap is less load-bearing — Googlebot can discover content via links alone.
Best practices: keep individual sitemap files under 50,000 URLs and 50 MB (split into multiple sitemap files for larger sites, with a sitemap index at /sitemap.xml). Include only canonical URLs that should be indexed (no redirects, no noindex pages). Reference the sitemap in robots.txt with a "Sitemap: https://example.com/sitemap.xml" line.
For AEO specifically, sitemap.xml does NOT replace llms.txt. The two serve different purposes: sitemap.xml is exhaustive (every indexable URL) for traditional search engines; llms.txt is curated (top 20-50 authoritative URLs) for AI engines. Ship both, do not conflate them.