A type of
unix created by a company after they have made changes from a basic standard like
BSD or
System V. This operating system then is usually only able to run on specific
hardware sold by the company that made these changes (
Sun,
HP,
IBM, and
SGI do this), or the operating system is sold for use on third party hardware (
SCO does this).
Contrast this to something like
FreeBSD or
Linux.