Unix

Unix (frequently typed as "UNIX"), today, is a catch-all name that may be used to describe any of a number of operating systems which are POSIX-compliant to some degree. While the original UNIX was developed at Bell Labs, the name today is applied to many of its direct descendants, as well as several operating systems and kernels which were inspired by it.

Note that an OS doesn't need to be fully POSIX-compliant to be considered Unix-like. The reverse is also true: a system can be POSIX-compliant without being Unix-like. Wikipedia has a brief discussion of Unix vs Unix-like here.