An error message that means the requested URL was not found on a particular server. This in turn means the Web page has either moved to a new address or is no longer being maintained online. It's also possible that you typed the address incorrectly or were given an incorrect address to begin with. If you click on an intriguing link listed by a search engine but get this message instead, you may still be able to figure out where the page moved. First, try putting your cursor at the end of the address in the URL box and backspace until you get to the root address (just after the .com or .net, for example). Then try finding it from the Web page at the root address.
Inspired by this error message, the term "404" is sometimes used for someone who's absolutely clueless, as in, "There's no use asking him; he's 404, dude."