A "404 error code" is an HTTP status code that indicates a webpage could not be found on a server. This error occurs when a user tries to access a URL that does not exist, either because the page has been deleted, the URL was typed incorrectly, or the link is broken. It is a common issue encountered while browsing the internet. When a 404 error occurs, the server responds with a message that typically includes the phrase "404 Not Found." Many websites customize their 404 error pages to provide helpful information or links to other parts of the site, improving user experience despite the error.