An "HTTP 404" is a standard response code in the Hypertext Transfer Protocol (HTTP) that indicates a requested resource could not be found on the server. This typically occurs when a user tries to access a webpage that has been moved, deleted, or never existed. The server returns this status code to inform the user that the specific URL they entered is not available. When a browser encounters an HTTP 404 error, it usually displays a message like "404 Not Found." This message helps users understand that the issue lies with the requested page, not their internet connection. Webmasters can customize these error pages to provide helpful navigation options or links to other sections of the website.