1. A dive in which the front of the body hits flat against a surface, especially of water: Celia did a belly flop into the swimming pool.
2. A shallow dive in which the front of the diver's body hits the water first: The children, supervised by the adults, were playing at the pool and did belly flops into the water, laughing hysterically all the time.

To execute a poorly done dive in which the front of the body lands flat on the surface of the water: Orville belly-flopped into the swimming pool right after his sister.