Tide

tide, any of the cyclic deformations of one astronomical body caused by the gravitational forces exerted by others. The most familiar are the periodic variations in sea level on Earth that correspond to changes in the relative positions of the Moon and the Sun. The tides may be regarded as forced waves, partially running waves and partially standing waves. They are manifested by vertical movements of the sea surface (the height maximum and minimum are called high water [HW] and low water [LW]) and alternating horizontal movements of the water, the tidal currents. The words ebb and flow are used to designate the falling tide and the rising tide, respectively.