Pronunciation: (kan'un-bôl"),[key] —n.Also,can'non ball". 1. a missile, usually round and made of iron or steel, designed to be fired from a cannon.
2. Tennis.a served ball that travels with great speed and describes little or no arc in flight.
3. anything that moves with great speed, as an express train.
—adj. 1. made from a curled-up position with the arms pressing the knees against one's chest: a cannonball dive. 2. moving at great speed: a train known as a cannonball express.