POSIX (Portable Operating System Interface) is a family of standards for maintaining compatibility between operating systems. POSIX specifies a set of interfaces (functions, headers, and command line utilities) that provide a uniform environment for software development.

It includes standards for a wide range of functionality, including file and directory operations and defines the command-line interface and the behavior of utilities such as cp, mv, and ls. It was designed to be independent of any particular operating system so that programs written in accordance with POSIX standards can be run on any POSIX-compliant system.