Short version: A 404 page is the page a visitor sees when the requested URL cannot be found on the server.
In practical website work, it usually means a link is broken, a page was deleted, a slug changed, or someone typed the address incorrectly. A 404 response does not automatically mean the page is gone forever; if content was permanently removed, a 410 status can be more specific. For users and SEO, the important part is what happens next: the page should make the situation clear and help people continue instead of leaving.
A good custom 404 page should keep the brand experience calm, offer useful navigation, and guide people toward search, key services, recent content, or the homepage. In a cleanup or migration, repeated 404s should be tracked and fixed with the right Redirect plan, especially when old URLs had traffic, links, or ranking value.
Related terms: URL structure, Redirect, and SEO.