(1) device for mechanically increasing the pressure of a gas. (2) often described as being either open, hermetic, or semihermetic to describe how the compressor and motor drive is situated in relation to the gas or vapor being compressed. Types include centrifugal, axial flow, reciprocating, rotary screw, rotary vane, scroll, or diaphragm. 1. device for mechanically increasing the pressure of a gas. 2. specific machine, with or without accessories, for compressing refrigerant vapor.