Darwin is an open source POSIX-compliant computer operating system released by Apple Inc. in 2000. It is composed of code developed by Apple, as well as code derived from NeXTSTEP, BSD, and other free software projects.
In July 2003, Apple released Darwin under version 2.0 of the Apple Public Source License (APSL), which the Free Software Foundation (FSF) approved as a free software license. Previous releases had taken place under an earlier version of the APSL that did not meet the FSF's definition of free software, although it met the requirements of the Open Source Definition.
The first version of the Apple Public Source License was approved by the Open Source Initiative (OSI). Version 2.0, released July 29, 2003 conforms to the Free Software Foundation guidelines, and is also approved by the OSI. The Free Software Foundation approved the Apple Public Source License 2.0 as a free software license and say it is acceptable for developers to work on projects that are already covered by this license. They recommend, however, that developers do not release new projects under this license, because the partial copyleft is not compatible with the GNU General Public License and allows linking with files released entirely as proprietary software. The license does, however, require that if any derivatives from the original source are released externally, that the source be made available.