Officially an operating system is not allowed to be called a Unix until it passes the Open Group's certification tests, and supports the necessary API's.

Nobody has yet stepped forward to pay the large fees that certification involves, so we're not allowed to call it Unix. Certification really doesn't mean very much anyway. Very few of the commercial operating systems have passed the Open Group tests.

Unofficially, Linux is very similar to the operating systems which are known as Unix, and for many purposes they are equivalent. Linux the kernel is an operating system kernel that behaves and performs similarly to the famous Unix operating system from AT&T Bell Labs. Linux is often called a "Unix-like" operating system. For more information, see http://www.unix-systems.org/what_is_unix.html.