Whitecapping in ocean surface waves
Whitecapping refers to the steepness-induced wave dissipation in deep water during which some air is entrained into the near-surface water, forming an emulsion of water and air bubbles (foam) that appears white. It occurs when the velocity of individual water particles near the wave crest exceed the phase speed of the wave, causing the front face of the wave to become too steep and "break". Steeper and more vigorous breakers are more efficient at entraining air bubbles into the water column, and they generate more foam.
Whitecapping is an essential process for air-sea gas exchange.

In the wave modeling community, because current spectral wave prediction models do not resolve air entrainment by the breaking waves, we use the term "whitecapping" to describe all steepness-induced deep-water wave dissipation. For example, see Hasselmann (1974).

Waves also break due to shoaling as they enter shallow water, and these are usually referred to as plunging breakers. While plunging breakers without doubt generate a lot of foam, we don't use the term whitecapping to describe them. Thus, whitecapping is a specific kind of wave breaking, as described in the first paragraph.

See a comprehensive review by Cavaleri et al. (2007) for a description of all the processes represented by current wave models.