Unstable (Debian)
In the Debian jargon, unstable is the development distribution of Debian. It is codenamed sid[1] (after the boy next door who broke toys in Toy Story). As unstable never becomes testing, this codename is permanent, contrary to the codenames of stable and testing, which have volatile codenames.
unstable has some newer packages, but suffers from an unusually high number of serious bugs, as many packages are new. In development periods, the number of bugs considered "Release-Critical" by the Debian project in unstable is typically between 2 and 3 times as many as the number in testing.
unstable is targeted at Debian developers for building packages to be uploaded to unstable, and other Debian users that know Debian well in general, master Debian package management, are good for reporting bugs, want to help the Debian project, and have some machines on which they don't depend.