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 "UnixLike" APIs, all called POSIX followed by some obscure serial number. Adhering to the PosixStandard is supposed to make a system sufficiently UnixLike that porting programs to it is easy. 

You can download the standard at http://www.opengroup.org/publications/catalog/t912.htm.

WindowsNt is POSIX-compliant. Enough said.  ''NT only implements the bare minimum (e.g. no sockets!) of the PosixStandard so as to be able to put a POSIX bulletpoint on the box.''