OS X’s kernel isn’t strictly Unix-based. It’s called XNU, and it’s a monolithic (non micro) hybrid of both Mach And BSD kernel components. What’s quite interesting is how many people (including myself up until a little while ago) think that OS X has a pure Unix (BSD) kernel.

But that’s not the case. In fact, XNU is actually an acronym for “X is Not Unix”.

In other words, while Apple’s marketing team loves to play the Unix card at every turn, its developers explicitly made a distinction between the OS X’s kernel and a true Unix kernel.

Wikipedia goes on to mention, however, that this distinction will become far less dramatic in the event that OS X becomes POSIX compliant in Leopard.