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operating system, standard
(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."