A hydroplane is an aircraft that's designed to land on water, or a boat that skims lightly across the water's surface. When you hydroplane, it's more likely to be in a car that's slipping on a wet road.
The noun hydroplane always refers to a vehicle, either a speedboat that seems to hover over the surface of water as it travels, or a plane that can take off and land on an ocean or lake. Used as a verb, hydroplane describes what these vehicles do — or what a fast-moving car does on a very wet surface, losing traction as it begins to float and slide. The prefix hydro- means "water" in Greek.