cannonball

Short definition from Wiktionary

Noun

1. 1
   A spherical projectile fired from a smoothbore cannon.
2. 2
   An explosive-filled hollow iron sphere fused through a hole and intended to explode at a calculated distance rather than explode on impact.
3. 3
   The act of jumping (typically into a swimming pool) with the legs bent and the arms wrapped around the knees to create a large splash, mimicking the flight and shape of a cannonball.
4. 4
   Something that moves fast, especially a fast train.
5. 5
   A served ball that travels with great speed and describes little or no arc in flight.

Verb

1. 1
   To jump or dive into water, performing a cannonball landing.
2. 2
   To career; to move rapidly.