“404” can refer to several things, so I need a little more context to understand what you’re interested in. Here are some possibilities:

HTTP Error Code:

In the world of web browsing, “404” is most commonly known as an HTTP error code. It indicates that the server could not find the requested resource (page, file, etc.) despite a valid request being sent. You might see this message if you:

• Typed the wrong URL
• Followed a broken link
• Tried to access a page that has been removed