Source : Free On-Line Dictionary of Computing

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."
     
        (1997-12-04)