The dynamic deformation of the spherical shape of a rotating celestial object brought about by the gravitational attraction of another, nearby, body. On Earth we experience highly visible changes in sea level twice a day because our planet and satellite Moon revolve around their mutual centre of gravity. Because the moon is 81 times lighter than the Earth that centre of gravity is located inside the Earth so the outward centrifugal force on the side of the Earth away from the moon causes the tidal bulge. The tides result in friction that slows the rotation of the Earth.
The vertical component of the particulate motion of a tidal wave. Although the accompanying horizontal movement of the water is part of the same phenomenon, it is preferable to designate this motion as tidal current.