Definition
noindex is the directive that excludes a page from search/AI engine indexes. Declared as <meta name='robots' content='noindex'> or HTTP header X-Robots-Tag: noindex. Crawlers still fetch the page to see the directive but don't add it to their index. Used for staging environments, internal admin pages, thank-you pages, paginated duplicates.
<meta name='robots' content='noindex'> on /admin → Googlebot crawls, sees directive, doesn't index. GPTBot honors the same directive.
How to optimize
Use noindex on: staging URLs, internal tools, thin auto-generated pages, thank-you pages, paginated duplicates beyond page 1. Don't combine noindex with robots.txt disallow, because the bot can't see the directive if it can't fetch.