The Portable Operating System Interface (POSIX) is a family of standards specified by the IEEE Computer Society for maintaining compatibility between operating systems.
POSIX defines the application programming interface (API), along with command line shells and utility interfaces, for software compatibility with variants of Unix and other operating systems. - WikiPedia
Enables you to use threads, real-time and inter-process communication (IPC).
Note that POSIX also sets the standards for signals, mutexes, scheduling etc.