The Single UNIX Specification is a set of standard documents maintained by the Austin Group and published by [http://www.opengroup.com The Open Group]. The current version (3) can be found at http://www.unix.org/version3/.
Check out the Wikipdia article on [http://en.wikipedia.org/wiki/Single_Unix_Specification The Single UNIX Specification].

The online version of the documents:
 * The Base Specifications http://www.opengroup.org/onlinepubs/009695399/toc.htm
 * ...


PS. The UNIX trademark is registered to The Open Group (see http://www.unix.org/ and http://www.unix.net/).