Indexability
The ability of a web page to be stored in a search engine's index and shown in search results, determined by technical factors and rules.
Indexability is the property of a web page that determines whether search engines are able to and willing to store it in their index for retrieval in search results. A page that is indexable is one that Googlebot can crawl, parse, and add to Google's index without being blocked by any rule.
Indexability is different from ranking. A page can be fully indexable and still rank poorly (or not at all for a given query) because of weak content, low authority, or poor relevance. Conversely, a page that is not indexable will never appear in search results, regardless of how well-optimized it is.
The main factors that block indexability: a noindex meta tag (<meta name="robots" content="noindex">), a noindex HTTP header (X-Robots-Tag: noindex), exclusion by robots.txt (the URL is blocked from crawling, so it cannot be indexed), a canonical tag pointing to a different URL, login walls (the page requires authentication), or a server that returns an error code (4xx, 5xx) when crawled. The most common cause by far is accidentally adding noindex to pages that should be indexed, often via a CMS setting, a staging environment that was never flipped to production, or a faulty SEO plugin.
To check whether a specific page is indexable, use Google Search Console's URL Inspection tool. It shows the indexability status, the canonical, and any blocking rule. For a site-wide audit, tools that integrate with the Google Search Console API can surface all non-indexable URLs grouped by reason.