14 C.F.R. § 135.601
(b) Definitions. For purposes of this subpart, the following definitions apply:
(1) Helicopter air ambulance operation means a flight, or sequence of flights, with a patient or medical personnel on board, for the purpose of medical transportation, by a part 135 certificate holder authorized by the Administrator to conduct helicopter air ambulance operations. A helicopter air ambulance operation includes, but is not limited to—