A meta directive that tells search engines to crawl a page but exclude it from the search index — used to suppress thin, duplicate, or private pages.
noindex in plain English
The noindex directive is implemented as either a meta tag (<meta name="robots" content="noindex">) in the page <head>, or an HTTP header (X-Robots-Tag: noindex). Search engines crawl the page, see the directive, and remove the URL from their index (or never add it). The page can still link out, the page can still be fetched by users — it's just hidden from search results.
Compare with robots.txt Disallow, which blocks crawling entirely. The difference matters: a Disallow'd URL that someone else links to can still appear in search results as a bare URL with no description (because Google won't crawl it to read the content). A noindex'd URL gets fully removed because Google did crawl it and saw the directive. For "please don't show this in search," noindex is the right tool. For "please don't even crawl this," Disallow is right (with the caveat that Disallow is a request, not a guarantee).
Standard ecommerce uses for noindex: thank-you pages (post-checkout confirmations), search results pages (your site's own internal search), filtered category permutations (color=red+size=large+brand=nike), tag archives that duplicate category content, draft posts, account pages, password-reset pages, and printer-friendly versions of regular pages. Each is a case where the page needs to exist for users but contributes nothing to your search presence and may dilute it.
A common mistake is noindex'ing the canonical version of a page accidentally — usually through a CMS setting or a misconfigured template that adds noindex to all pages of a certain type. Audit your live site with Screaming Frog or Sitebulb periodically and review every URL flagged as noindex'd. Anything that should be ranking but isn't getting traffic is the first place to check.
Why noindex matters for ecommerce
For ecommerce, noindex is the cleanup tool that keeps your indexed footprint focused. Stores with hundreds of filter-permutation URLs in Google's index dilute their topical authority — a hundred near-duplicate pages compete with each other rather than concentrating ranking signals. Strategically noindex'ing the noise (filter combinations, search pages, tag pages) lets your real category and product pages do their job. Combine with proper canonical tags to maintain link equity flow.