Disturbances that travel across the ocean surface, transferring energy without a net transport of water, are waves. Most ocean waves are generated by wind. Their size depends on wind speed, duration, and the distance over which the wind blows—the fetch.

A wave is characterized by its height, wavelength, and period. In deep water, wave speed increases with wavelength; longer waves outrun shorter ones and form swell. When waves enter shallow water they slow, shorten, and eventually break, delivering their energy to the shore. Waves shape beaches, drive coastal currents, and provide a renewable energy resource still under development.

Extreme waves accompany storms and can threaten ships and coastal structures. Statistical descriptions of the wave field, such as significant wave height, are used in forecasts and engineering design. Observations come from buoys, satellites, and coastal radars. Understanding wave dynamics is fundamental to coastal engineering and maritime safety.

Even a gentle swell on a calm day carries energy that began in a storm far across the ocean.