In hydrodynamics, the motion of a fluid particle induced by the passage of a progressive gravity wave. When the wave height is small and the fluid depth is great, the orbit is a circle the radius of which decreases exponentially with depth. In shallow fluids, the orbit is an ellipse, which degenerates into a horizontal line at the bottom boundary of the fluid.
In physics, orbital motion is the either a motion of a planet in a planetary orbit, or a motion of an electron around the nucleus of an atom, or any other motion of parts of a bound system. In quantum mechanics, orbital motion contributes to the angular momentum, but there are also other contributions such as spin.