What describes the flat surface forming the stern of a vessel?

The term that describes the flat surface forming the stern of a vessel is the transom. The transom is a crucial part of a boat's structure, specifically designed to provide stability and support at the rear of the vessel. It serves not only as the end of the hull but also as a mounting point for features such as the motor or rudder, depending on the design of the boat.

The freeboard refers to the distance from the waterline to the upper deck level, which is not related to the shape of the stern. The bow is the front part of the vessel, opposite to the stern, while the hull encompasses the entire body of the boat, excluding any superstructure. Hence, the transom is the most precise term to describe the flat surface at the back of a vessel.