The Apple standard library for the OSX operating system for routing MIDI and providing software access to MIDI interface hardware connected to the computer. In Apple operating systems prior to OSX, this function was performed by third-party supplied libraries, mainly MOTU's FreeMIDI and Opcode Systems' OMS. Some ex-Opcode employees had a hand in writing the CoreMIDI specifications and software for Apple.
There was a change in how device and interface names are manage in OSX 10.3. Code that uses CoreMIDI and was written for versions 10.2 or earlier will not work with 10.3 or later.