Mac OS X 10.5 Leopard has achieved UNIX 03 certification. The certification is granted by the Open Group who is an organisation that ensures conformity to recognised standards.
Ars Techinca notes that Apple has always been keen to tout Unix as the base for Mac OS X, playing up the security, stability, and compatibility that comes with its BSD foundation.
From a developer point of view, UNIX 03 certification means that Leopard conforms to the Single UNIX Specification Version 3 (SUS), which outlines how components like the shell, compiler, and C APIs should function.
"Any software written for the SUS specification is easily portable to a UNIX 03 operating system, meaning that enterprise customers who need a 'real' UNIX for their applications can now use Leopard servers if they so desire," writes Ars. "Leopard's certification also gives developers another option for a development platforms, which could translate into some extra Mac sales."