POSIX (Portable Operating System Interface) is an operating system interface standardized by ISO/IEC, IEEE and The Open Group.
There are a bunch of standards for "Unix Like" APIs, all called POSIX followed by some obscure serial number. Adhering to the Posix Standard is supposed to make a system sufficiently Unix Like that porting programs to it is easy.
You can download the standard at www.opengroup.org 
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Windows Nt is POSIX-compliant. Enough said. NT only implements the bare minimum (e.g. no sockets!) of the Posix Standard so as to be able to put a POSIX bulletpoint on the box.
See original on c2.com