From The Free On-line Dictionary of Computing (19 January 2023):
Portable Operating System Interface
POSIX
<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."
(1997-12-04)