A cannon is a weapon that uses a chemical explosive to propel a massive projectile out of a tube at high velocity. A cannon is distinguished from a firearm by being too large for a single person to move and wield.
Cannons in Science Fiction
Large directed-energy weapons used by advanced civilizations are often called cannons, such as phaser cannons or blaster cannons. As with real weapons, the distinction between a small arm and a cannon is mostly size.