404 Error.

A 404 Not Found error occurs when a server cannot locate the resource at the requested URL. The page may have been deleted, moved without a redirect, or the URL may have been mistyped. The server receives the request successfully but has no content to return, so it responds with a 404 status code.

SEO Impact of 404 Errors

404 errors on pages with inbound backlinks waste link equity—the links still point to the dead URL rather than benefiting any live page. Google will eventually de-index 404 pages, but this can take months. 404 errors on pages with no links or traffic are low priority; those with backlinks or historical organic traffic should be 301 redirected to the most relevant live alternative.

Soft 404s vs Hard 404s

A hard 404 returns HTTP status code 404 in the response header. A soft 404 returns HTTP 200 OK but displays a 'page not found' message—confusing crawlers into indexing an empty or useless page. Soft 404s are harder to detect and require fixing both the page content and the HTTP response code to correctly signal non-existence to search engines.