Short for "Portable Operating System Interface for uni-X", POSIX is a set of standards codified by the IEEE and issued by ANSI and ISO. The goal of POSIX is to ease the task of cross-platform software development by establishing a set of guidelines for operating system vendors to follow. Ideally, a developer should have to write a program only once to run on all POSIX-compliant systems. Most modern commercial Unix implementations and many free ones are POSIX compliant. There are actually several different POSIX releases, but the most important are POSIX.1 and POSIX.2, which define system calls and command-line interface, respectively.

The POSIX specifications describe an operating system that is similar to, but not necessarily the same as, Unix. Though POSIX is heavily based on the BSD and System V releases, non-Unix systems such as Microsoft's Windows NT and IBM's OpenEdition MVS are POSIX compliant.

For more information about POSIX, visit ISO's POSIX page or the page of the Portable Application Standards Committee (PASC), the IEEE group responsible for POSIX.

There are other standardization efforts in the Unix world. The most prominent are the Open Group XPG guidelines and the System V Interface Definitions (SVID). POSIX, however, is the only major standard endorsed by the vendor-neutral standards organizations.