Darwin is a BSD operating system, released by Apple in 2000. It is composed of code developed by Apple, as well as code derived from NeXTSTEP, BSD, and other free software projects.
Darwin forms the core set of components upon which Mac OS X and iOS are based.
Mach Kernel was developed at Carnegie Mellon University, from 1985 to 1994. NeXT created a derivative of Mach, XNU OS. NeXT teamed up with Sun Microsystems to develop OpenStep, a cross-platform object-oriented API standard. Apple Computer acquired NeXT for $429 million in 1996. In 1997, Apple released an OS with internal codename Rhapsody OS, only for developers. Apple then released Mac OS X Server 1.0 in 1999, Mac OS X Public Beta in 2000, and Mac OS X 10.0 in 2001. Also in 2000, the core operating system components of Mac OS X were released as open-source software under the Apple Public Source License (APSL) as Darwin; but the higher-level components, such as the Cocoa and Carbon frameworks, remained closed-source.