POSIX ( /ˈpɒzɪks/ POZ-iks), an acronym for "Portable Operating System Interface", is a family of standards specified by the IEEE 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.

Originally, the name "POSIX" referred to IEEE Std 1003.1-1988, released in 1988. The family of POSIX standards is formally designated as IEEE 1003 and the international standard name is ISO/IEC 9945.

The standards, formerly known as IEEE-IX, emerged from a project that began circa 1985. Richard Stallman suggested the name POSIX in response to an IEEE request for a memorable name.

The POSIX specifications for Unix-like operating system environments originally consisted of a single document for the core programming interface, but eventually grew to 19 separate documents (for example, POSIX.1, POSIX.2 etc) [1]. The standardized user command line and scripting interface were based on the Korn shell^{[citation needed]}. Many user-level programs, services, and utilities including awk, echo, ed were also standardized, along with required program-level services including basic I/O (file, terminal, and network) services. POSIX also defines a standard threading library API which is supported by most modern operating systems. Nowadays, most of POSIX parts are combined into a single standard, IEEE Std 1003.1-2008, also known as POSIX.1-2008.