POSIX is a set of specifications, referring to the IEEE 1003 family of standards. POSIX is a trademark of the IEEE. The term POSIX.n is used to refer to IEEE Std. 1003.n in its latest revision. When the term POSIX is used without a suffix, it almost always refers to the POSIX.1 standard.
The POSIX.1 standard is composed of four parts:
The POSIX.1 standard is also known as the Single Unix Specification (SUS), which is maintained by the Open Group. Today, "UNIX", "POSIX.1", the "Single Unix Specification", "IEEE Std. 1003.1" and "ISO/IEC 9945" all refer to the same document.