HTTP codes
Not found
The server has no page at the address that was requested.
Also called: not found, page not found, broken link, dead link
404 is the status code a server answers with when there is no page at the address requested. Every site returns them, and that on its own is not a problem.
The address may be wrong, or the page may have moved. Either way the rest of the site keeps working normally.
Where you meet it
A site that is kept up redirects its old addresses, so visitors land on the new page instead of a dead end. Skip that during a redesign and every link anyone ever shared stops working.
Related concepts