It isn't Unix unless it's Unix 03 certified.
Apple's upcoming OS X 10.5 "Leopard" release has officially gained certification as a Unix operating system by the Open Group.
The Unix 03 certification was awarded on 18 May.
In order to achieve the certification, an operating system must comply to the ISO and IEEE standards for Unix operating systems.
The standard was developed in the 1980's to define a Unix specification that would allow software to function across various proprietary Unix versions.
The certification means that Apple can now brand Leopard simply as Unix. Previously, OS X releases had been described by the company as "Unix-like," leaving some room for doubt as to what extend the software adhered to Unix specifications and how compatible certain software would be.
Other Unix 03 certified operating systems include Sun's Solaris, Hewlett Packard's HP-UX, and IBM's AIX.
Leopard is slated for release by October.