Mac OS X, pronounced "Mac Oh-Es Ten," is the current version of the operating system used on Apple Macintosh computers. If you happen to pronounce it "Mac OS X," computer nerds and dedicated Mac users will be quick to correct you. While the name may be a bit confusing, Mac OS X is an advanced, user-friendly operating system.

Previous versions of the Mac OS, were based on the original Macintosh operating system, released in 1984. In the late 1990's, many computer users felt Windows had "caught up" to the Mac OS and Apple's operating system began to appear a bit dated. So Apple completely revamped the Mac OS and created a new operating system from the ground up.

While much of the code used to build Mac OS X was written from scratch, a lot was taken from the NEXTSTEP operating system. NEXTSTEP was a Unix-based system that ran on NeXT computers, which are no longer in production. NeXT was acquired by Apple in 1997 and Steve Jobs was hired as interim CEO. Apple developers took the Unix-based code from NEXTSTEP and combined it with the graphical user interface (GUI) of Mac OS 9. The result was a stable, high-performance operating system that had the stability of Unix and the intuitive interface of the Macintosh. Mac OS X 10.0 was released in 2001.

Since the initial release, Apple has released several major updates to Mac OS X, at a pace of roughly one update a year. The list of OS X versions include 10.0 Cheetah, 10.1 Puma, 10.2 Jaguar, 10.3 Panther, and 10.4 Tiger. Mac OS X 10.5 is expected to be called Leopard.

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Definition from the PC Glossary