A "404 Error" 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 moved, deleted, or the URL was typed incorrectly. When a 404 Error appears, users typically see a message stating that the page is not available. Web developers can customize these error pages to provide helpful links or suggestions, improving user experience. Commonly, this error is associated with websites and can be encountered on any browser or device.