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.