Wave
Trough
The lowest point of the wave at the base of the face, where there is little power or momentum.
The trough is the bottom of the wave, the low point between the face and the flats. It holds very little of the wave's energy, so riders use it mainly as the bottom of a bottom turn or the low point of a pump before driving back up the face toward the lip.
Understanding the trough versus the lip and pocket helps a rider read where the power is and time their carves and pumps to stay connected to the wave's push.