|(operating system, standard)||Portable Operating System Interface - (POSIX) A set of IEEE standards
designed to provide application portability between Unix
variants. IEEE 1003.1 defines a Unix-like operating system
interface, IEEE 1003.2 defines the shell and utilities and
IEEE 1003.4 defines real-time extensions.|
["More UNIX For Dummies", ISBN: 1-56884-361-5] says it stands for "Portable Operating System Interface with an X thrown in to make it sound cooler."