The Colonial Military Administration, believed to also be called Central Command, abbreviated as CENTCOM, is the organization that commands all military forces of the UNSC. It is apparently above HIGHCOM. They personally ordered Project: ORION as well as the creation of the Spartan supersoldiers, to take place.
NOTE: The Colonial Military Administration has only been mentioned once, its functions are unclear. But it is likely under the auspecies of the Colonial Administration Authority.