The Chief Medical Officer, or CMO, was asenior staff position within Starfleet.
Most CMO's ran the medical department aboard a starship, space station or planet based Starfleet. They also served on the staffs of Fleets and Admiral's. The head of Starfleet Medical was also referred to as the Chief Medial Officer of Starfleet.
The United Federation Marince Corps also had Chief Medical Officer positions, as they are medical personnel were borrowed from Starfleet.