Debian Long Term Support
Debian Long Term Support (LTS) is a project to extend the lifetime of all Debian stable releases to (at least) 5 years. Debian LTS is not handled by the Debian security team, but by a separate group of volunteers and companies interested in making it a success.
Thus the Debian LTS team takes over security maintenance of the various releases once the Debian Security team stops its work.
For more information see the wiki page about LTS.