V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

POSIX
       Portable Operating System for unIX (OS, IEEE 1003, ISO 9945,
PASC, Unix)


The Free On-line Dictionary of Computing (30 December 2018):

Portable Operating System Interface
POSIX

    (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."

   (1997-12-04)