emission noun emis??sion \??-??mish-??n\ (Medical Dictionary)Medical Definition of EMISSION 1: an act or instance of emitting 2a : something sent forth by emitting: as (1) : electrons discharged from a surface (2) : electromagnetic waves radiated by an antenna or a celestial body (3) : substances and especially pollutants discharged into the air (as by a smokestack or an automobile gasoline engine