Extended definition
HTML tag (<meta name="robots" content="noindex">) telling Google NOT to index the page. Useful for test pages, duplicate pages, form thank-you pages.
Context and application
Important difference from robots.txt: noindex allows crawl (Googlebot READS the page to see the tag) but forbids indexing. For administrative pages (login, password reset, thank-you, internal admin), noindex is the only correct solution. On staging URLs, the ideal combination is noindex + HTTP basic auth — not just robots.txt (which, as we saw, guarantees nothing).
Related terms
Indexing, Robots.txt.