hydro- + aeroplane (hydro- + aero- + plane)
hydroaeroplane (plural hydroaeroplanes)
- (aviation, obsolete) a floatplane, an airplane specialized for operating on water, having pontoon floats instead of skids or wheels
(obsolete aviation term):
This term was used to describe the very first seaplanes and floatplanes, at the dawn of powered aviation through early-post-WWI era (late 'teens, early twenties).