POSIX

POSIX (Portable Operating System Interface) is a set of IEEE standards designed to provide application portability between Unix variants.

Definition
IEEE 1003.1, first published on August 22, 1988, defined a Unix-like operating system interface. This standard has been updated as IEEE 1003.2 defined the shell and utilities, and IEEE 1003.4 defined  extensions.

Etymology
According to More UNIX For Dummies the acronym was created from "Portable Operating System Interface with an X thrown in to make it sound cooler."