301 redirect
A permanent redirect from an old address to a new one, so that visitors and search engines find a page that has moved.
Glossary · Websites & e-commerce
301 is an HTTP status code meaning “moved permanently”. When a page’s address changes, the server answers the old address with a 301 pointing at the new one, and the browser takes the visitor there automatically.
For search engines it is the signal that the old address is retired and that what it had earned, links from other sites and its place in search results, belongs to the new one. Without a redirect the old address returns a 404 error and that value is lost.
Redirects are most often forgotten during a redesign. That is why a list of every old address is drawn up before launch, each with the place it will lead. Redirect chains (A leads to B, B to C) slow a site down, so each old address should lead straight to its final destination.