- A powerful, multitasking, multi-user OS that serves as the basis for many modern OS
- multitasking: can run multiple applications at the same time
- multi-user: can have different permission levels active at the same time
- was popular because it was written in C and was easily applicable to other computers
- Many operating systems wanted to copy unix ⇒ POSIX
- contained rules to be considered as another Unix version
- MacOS and IOS are both official Unix OS
- first owned by AT&T
- which means proprietary << led to development of The GNU project