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