• (a.) Pertaining to, or depending upon, both liquid and gaseous substances; as, hydropneumatic apparatus for collecting gases over water or other liquids.
Hy`dro·pneu·mat'ic adjective [ Hydro- , 1 + pneumatic : confer French hydropneumatique .] Pertaining to, or depending upon, both liquid and gaseous substances; as, hydropneumatic apparatus for collecting gases over water or other liquids. 
hydropneumatic Operated by both water and air power.
A water system, usually small, in which a water pump is automatically controlled by the pressure in a compressed air tank.