POSIX - Portable Operating System Interface X ???
See http://www.unix.org/version3/iso_std.html for some pointers to POSIX.
Anyone else have pointers to useful online resources relating to POSIX?
POSIX is not just one standard, but a series of standards defining APIs, command line interfaces, etc.. There is a standard relating to the operating system itself. There is at least one standard relating to utilities. There is a standard relating to real time programming interfaces, etc.