Portable Operating System Interface is the name of a family of related standards specified by the IEEE to define the application programming interface (API), along with shell and utilities interfaces for software compatible with variants of the Unix operating system, although the standard can apply to any operating system.

The POSIX specifications for user and software interfaces to an operating system originally consisted of 17 separate documents.

The standardized user command line and scripting interface were based on the Korn shell. 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, 10 out of these 17 parts are combined into a single standard, IEEE Std 1003.1-2008, also known as POSIX:2008.