301

 301  (301 Moved Permanently) is an HTTP status code returned from a web server to indicate the URL is permanently redirected to another URL.

In general, a redirect is a way to send both users and search engines to a different URL from the one they originally requested. The three most commonly used redirects are 301, 302, and Meta Refresh. These are frequently used/needed when changing CMSes and potentially the structure of one's permalinks to get the old URLs to redirect to the new ones.

Here's a useful gist for creating 301 redirects within an .htaccess file for sites using Apache : https://gist.github.com/ScottPhillips/1721489