POSIX (Portable Operating System Interface) is a set of standard operating system interfaces based on the Unix operating system. The need for standardization arose because enterprises using computers wanted to be able to develop programs that could be moved among different manufacturer's computer systems without having to be recoded. Unix was selected as the basis for a standard system interface partly because it was "manufacturer-neutral." However, several major versions of Unix existed so there was a need to develop a common denominator system.
Informally, each standard in the POSIX set is defined by a decimal following the POSIX. Thus, POSIX.1 is the standard for an application program interface in the C language. POSIX.2 is the standard shell and utility interface (that is to say, the user's command interface with the operating system). These are the main two interfaces, but additional interfaces, such as POSIX.4 for thread management, have been developed or are being developed. The POSIX interfaces were developed under the auspices of the Institute of Electrical and Electronics Engineers (IEEE).
POSIX.1 and POSIX.2 interfaces are included in a somewhat larger interface known as the X/Open Programming Guide (also known as the "Single UNIX Specification" and "UNIX 03"). The Open Group, an industry standards group, owns the UNIX trademark and can thus "brand" operating systems that conform to the interface as "UNIX" systems. IBM's OS/390 is an example of an operating system that includes a branded UNIX interface. (Note that the trademark is "UNIX"; the generic terms for these operating systems is "Unix.")