DebianReleases > Debian Sid
Debian/Sid is always the codename for unstable.
As with all Debian release names thus far, the Sid distribution takes its name from a ToyStory character. Commonly (and wrongly) acronymed as "Still In Development", Sid is the kid next door who breaks his toys and makes nasty creatures of them.
While other release code names (such as Potato or Woody) progress in time from being testing to being stable, Sid is forever doomed to being unstable; therefore, it will never be "released".
See also
Debian/Unstable (the other name of Debian/Sid).