Mac OS X is an operating system from Apple Inc. It was the successor to Mac OS 9 and here the 'X' means 10. Mac OS X is based upon the NeXTSTEP and OpenStep OSes developed in the 1980s and 1990s by NeXT Inc., which Apple purchased in 1996.

Mac OS X consists of a Mach/BSD-based kernel, userland primarily based on FreeBSD, and additional frameworks (written in C, C++ and Objective-C) providing user interface and application-level services.