Unix is one of the granddaddies of computer operating systems, born in 1971 on a PDP-11 in Bell Laboratories. The people who brought Unix to life include Ken Thompson, Dennis Ritchie, Brian Kernighan, Doug McIlroy, Rob Pike, and a host of others (whose names I'm forgetting at this edit -ArielMT). The Unix Wars of the 1980s very nearly killed it, but it's still doing well.
It survives as, among other forms, FreeBSD, NetBSD, and OpenBSD. (OpenBSD is often cited positively for its secure-by-default philosophy.) A notable Unix variant later turned to famously evil bent by Darl McBride is Unixware and related SCO Unix products.
Unix also inspired Minix, a Unix-like OS to teach operating system design, which in turn inspired Linux, the most popular Unix-like OS in use today.
Words like "Un*x" and "*nix" are used to indicate Unix and all Unix-like operating systems collectively.
One of the more notable things about Unix is the fact that the BSD Mach variant was used as the foundation of the NeXTStep OS used on NeXT computers, which in turn came to be used in OpenStep, Rhapsody, and finally Mac OS X, the OS running all Macintoshes since 2002.
Cards tagged Unix: