Trademark An IEEE standard (P1003) that defines the behaviour of a set of supervisor calls, basing these closely on those found in UNIX. However, POSIX is not itself an operating system so much as a formal description of one form of operating system of which UNIX is a specific instance. The intention is that a program written in such a way as to use only those functions defined by the POSIX specifications will be readily portable between different operating systems, provided that these are all conformant to the POSIX definitions.

http://standards.ieee.org/develop/wg/POSIX.html The IEEE POSIX Certification Authority