POSIX stands for Portable Operating System Interface for UniX. It is an IEEE 1003.1 standard that defines the language interface between application programs and the Unix operating system. Adherence to the POSIX ensures compatibility when programs are moved/migrated from one unix computer to another.
AnswerStrictly speaking, POSIX is an API specification, in conjunction with a specification of certain specific utility programs which must be present. While originally based on the UNIX Operating System, POSIX does
not have to mean a UNIX system. For example, both certain versions of Windows NT and IBM's OS/360 have received POSIX certification as complying with the specification.
As noted above, POSIX is about insuring compatibility of programs between systems, allowing a POSIX-compliant program to run on any POSIX-compliant Operating System.