What exactly is POSIX?

I see POSIX mentioned often and everywhere, and I had assumed it to be the baseline UNIX standard.. until I noticed the following excerpt on a Wikipedia page: [*The Open Group*](http://en.wikipedia.org/wiki/The_Open_Group) > The Open Group is most famous as the > certifying body for the UNIX > trademark, and its publication of > the *Single UNIX Specification > technical standard*, ***which extends > the POSIX standards and is the > official definition of a UNIX system***. If the official definition of a UNIX system is an extension of POSIX, then what exactly is POSIX? ,,, It surely seems to be a touchstone of the UNIX world, but I don't know how it fits into the overall picture.