belly flop

Short definition from Wiktionary

Noun

1. 1
   A style of diving into a body of water in which the surface impact is made mostly by one's abdomen.
2. 2
   A similar move performed on a trampoline.
3. 3
   A flop; an utter failure.
4. 4
   Alternative spelling of belly flop.

Verb

1. 1
   To perform this type of dive.
2. 2
   To perform this type of trampolining move.