Source: https://9to5mac.com/2014/10/31/apple-releases-os-10-10-yosemite-open-source-darwin-code/
Darwin is an open source UNIX-like 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.1)
Darwin forms the core set of components upon which OS X and iOS are based. It is mostly POSIX compatible, but has never, by itself, been certified as being compatible with any version of POSIX. (OS X, since Leopard, has been certified as compatible with the Single UNIX Specification version 3 (SUSv3)).