What characteristic defines a plunging wave?

A plunging wave is characterized by its steep, barreling shape, which occurs when the wave's crest moves faster than the base as it approaches shallow water. This steep incline causes the wave to curl over itself, forming a prominent tube or barrel shape. The energy of the wave is concentrated in a smaller area due to this steep gradient, resulting in a powerful and dynamic wave that can create excellent conditions for surfing. The barreling nature of plunging waves also makes them visually impressive as they break further from the shore, providing a distinctive and exciting experience for surfers.

In contrast, other types of waves have distinct characteristics that differentiate them from plunging waves. For example, a spilling wave spills gently in a less steep manner, often characterized by a gradual crest that breaks smoothly along the front. Low-energy waves do not significantly break at all and typically are more suited for activities like swimming rather than surfing. Waves that form consistently refer to the regularity and predictability of waves in certain locations, which does not specifically indicate the steep nature of a plunging wave. Thus, the defining attribute of a plunging wave is its steep, barreling shape, making it the correct answer to the question.