The length of the perimeter in contact with the water. For a circular pipe of inside diameter "d", flowing full, the wetted perimeter is the circumference, d. The same pipe flowing half full would have a wetted perimeter of d/2.
the wetted surface of a stream (culvert) cross section which causes resistance to flow. The water to surface interface is a distance, usually expressed in feet.
The term wetted perimeter is common in Civil Engineering and also in heat transfer applications. Associated with the hydraulic diameter. The term is simply defined using the example of a cross section of a river.