Noun

1. 1. A spherical projectile fired from a smoothbore cannon; a solid shot; a solid round shot; a ball. 2. An explosive-filled hollow iron sphere fused through a hole and intended to explode at a calculated distance rather than explode on impact.
2. The result of running and jumping in a flexed position into a swimming pool to create a large splash, mimicking the flight and shape of a cannonball.

   The cannonball could be called the S.U.V. of the pool — oversized, brash, hormonally hardwired.

   • The New Yorker, 30 August 2004, p.40

   I would call it a water-entry stunt, not a dive.

   • The New Yorker, 30 August 2004, p.40, quoting "Coach O'Brien"

3. Something that moves fast.

   Meetings of the model train club always begin with the song "Wabash Cannonball".

Verb

1. To jump/dive into water doing a cannonball landing.

   He cannonballed into the pool, drenching us all.