A movement of ocean water characterized by regularity, either of a cyclic nature or, more commonly, as a continuous stream flowing along a definable path. Three general classes, by cause, may be distinguished: 1) currents related to seawater density gradients, comprising the various types of gradient current; 2) wind-driven currents, which are those directly produced by the stress exerted by the wind upon the ocean surface; and 3) currents produced by long-wave motions. The latter are principally tidal currents, but may include currents associated with internal waves, tsunamis, and seiches. The major ocean currents are of continuous, stream-flow character, and are of first-order importance in the maintenance of the earth's thermodynamic balance.