or the angle of the bottom of the boat. A zero degree deadrise means a flat bottom boat, which you don't see very often. Higher numbers mean a sharper vee. In general, a flatter bottom makes boat plane easier, go faster and get better fuel use. A deeper vee makes the boat ride better, especially in choppy water, and provides more volume in the cabin.
This boat has zero deadrise:
http://www.butlerwebs.com/jokes/images/boat-table.jpg