noindex is a robots meta directive that tells search engines not to include a page in their index. Set via <meta name="robots" content="noindex"> in the page <head> or via the X-Robots-Tag HTTP response header. Pages with noindex still get crawled (so internal links are followed) but never appear in search results.
Common noindex use cases: thin pages with low standalone value (paginated archive pages, internal search results, filter combinations), per-user surfaces (dashboards, settings, account pages), staging/preview environments, and content scheduled for future publication.
For SEO, noindex is the right tool when a page exists for a non-search-engine reason but should not compete in SERPs. A common mistake is using noindex on category pages that should rank — those should be canonicalized or improved, not noindexed.
For AEO, noindex applies to AI engines too. Pages with noindex are excluded from AI crawler indexes by most major engines. This is the right outcome for per-user surfaces but the wrong outcome for thin marketing pages — fix the thinness rather than noindexing it.