The undulating movement of the surface of the open sea WordNet 3.0 & Open English WordNet

A swell, also sometimes referred to as ground swell, in the context of an ocean, sea or lake, is a series of mechanical waves that propagate along the interface between water and air under the predominating influence of gravity, and thus are often referred to as surface gravity waves. Wikipedia

A type of ocean wave Wikipedia Disambiguation

A formation of long wavelength ocean surface waves Wikipedia Disambiguation

A regular movement of marine waves created by wind stress in the open ocean which travels considerable distances. OmegaWiki

A long series of ocean waves, generally produced by wind, and lasting after the wind has ceased. Wiktionary

Series of waves, lasting after the wind has ceased. Wiktionary (translation)