the seaward, subsurface flow or draft of water from waves breaking on a beach.
any strong current
below the surface of a body of water, moving in a direction different from that of the surface current.
2. Undertow, underset, riptide are terms for a usually strong undercurrent in the ocean, contrary to the direction of surface water. Undertow and another nautical term, underset (a set or current contrary to the general set of the water, or contrary to the wind), came into notice early in the 19th century. The former is still in general use along the Atlantic coast; the latter now less well known. Rip in use in the U.S. by the late 18th century, properly means a violently disturbed place in a body of water, usually by the meeting of opposing tides. Of recent years, in the form riptide it has also been used, especially on the Pacific coast, to mean much the same as undertow dangerous to bathers where heavy surf prevails.