from
The Free On-line Dictionary of Computing (8 July 2008)
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)