A cannon is a large gun that fires heavy metal shells or other projectiles. Originally it was attached to wheels and pulled by men.
More recently, cannon refers to the largest gun on a tank or the automatic guns on a plane. All cannons, however old they are, have basically the same shape — a long, thick tube. Not surprising, then, that the term comes from the Latin word canna, meaning "tube" or "reed." Do not confuse with the word canon, with one n, which is a term of classification, as in its most common usage, "the literary canon."