Just as there are many varieties of UNIX, there are many UNIX standards:

• Probably the oldest standard that people still refer to is AT&T's 1985 System V Interface Definition (SVID). This standard shows up, for instance, in man pages describing the standards compliance of functions that have been in the C library "forever."
• Meanwhile, X/Open (now the Open Group) was developing "portability guides" with names like XPG2, XPG3, and so on. XPG1 was actually released in 1995. The XPG guides are largely subsumed into newer specs, but once again, are still referred to sometimes in documentation.
• The IEEE's POSIX standard showed up in 1990 with updates in 1992 and 1993 and a second edition in 1996. It's still a viable standard, although it has suffered from poor accessibility. POSIX specs have names like 1003.x; for instance, 1003.1 and 1003.2, which refer to different parts of the standard, or 1003.1-1988 and 1003.1-1990, which refer to two versions of the standard.
• The fairly ominous sounding "Spec 1170" (also known as "UNIX 98" or "Single Unix Specification") is probably the most complete specification; it is produced by the Open Group, and is effectively a descendant of the XPG series. In practice, this is "the" UNIX standard these days, although it's a little large; this has had an impact on conformance testing.
• The Linux Standards Base is not strictly a UNIX standard, but it's a standardization effort relevant to a very large number of developers working with code designed to run "on UNIX."