The Collaborative International Dictionary
Hydropneumatic
Hydropneumatic \Hy`dro*pneu*mat"ic\, a. [Hydro-, 1 + pneumatic: cf. F. hydropneumatique.] Pertaining to, or depending upon, both liquid and gaseous substances; as, hydropneumatic apparatus for collecting gases over water or other liquids.
Wiktionary
hydropneumatic
a. 1 Operated by both fluid and air power 2 Being, or pertaining to, a type of automotive suspension system based on a nitrogen springing medium.