What is Noindex?
Noindex is a directive that tells search engines not to include a specific page in their search results index. Implemented via meta robots tag or X-Robots-Tag HTTP header.
When to use noindex: duplicate content pages, internal search results, admin/login pages, thank-you/confirmation pages, paginated archives (sometimes), and thin content pages you want to keep but not rank.
When NOT to use noindex: any page you want to appear in search results. Accidental noindex on important pages is one of the most common (and devastating) SEO mistakes found in audits.
Important: noindex means 'don't show this in search results.' It does NOT mean 'don't crawl this' (use robots.txt for that). Google still crawls noindexed pages to find links.
Example
A staging site accidentally deployed to production with noindex on all pages would disappear from Google entirely. This is caught by audit tools like Vantacron as a critical issue.