1. A gravity wave formed on the free surface of a fluid. In classical hydrodynamics, to distinguish surface waves from tidal waves, the condition is imposed that vertical accelerations are not negligible. Dynamically, this wave is similar to that on an interface separating two fluids, becoming identical in the case of zero density in the upper fluid. 2. See ground wave. 3. A wave at the interface of a light and a much heavier fluid, in particular of the air–sea interface. See surface gravity wave.