swell

A regular movement of marine waves created by wind stress in the open ocean which travels considerable distances away from the generating field and into another wind field. The waves are characterized by relatively smooth, generally unbroken, crests and a fairly regular wavelength, but swell increases in wavelength and decreases in wave height as it moves away from the generating area. Local wind waves may be superimposed upon swell waves as they approach a coastline, thereby creating sharper crests and a choppy sea. (Source: WHIT)