UNIX
A family of multi-user operating systems, first developed by AT&T Bell Laboratories in the 1970s and then licensed to many universities.
UNIX is designed to be used by many people at the same time (it is multi-user) and has TCP/IP built-in. It is the most common operating system for servers on the Internet.
UNIX has numerous variants including IRIX (SGI), Solaris (Sun), and derivatives including Linux, Apple OSX, and others.