The Single UNIX Specification is an industry standard description of the C language program and user command interfaces for a standard Unix operating system. A Unix operating system that conforms to The Single UNIX Specification is allowed to use the UNIX trademark. The Single UNIX Specification was developed to ensure that a program developed in one Unix operating system would run in a somewhat different Unix operating system. The specification is owned by The Open Group, an industry group which oversees UNIX certification and branding.
The latest version of the Single UNIX Specification is also known as UNIX 03.
Note that most trade publications (including TechTarget and whatis.com) use the mixed case "Unix" to refer generally to any of the various operating systems that evolved from the original one. The Open Group's industry standard operating system and trademark for branding is the all upper-case "UNIX."