A big damn gun; classified by the weight of the shot it fires. (For comparison, very small, light cannons were three- or six-pounders.) See also 32-pounder.
I have seen a twenty-four pounder fired with just powder (no shot, of course)--it is an impressive size, and quite loud and frightening--they were used as siege cannon at Yorktown in 1781, and were about the largest guns used during the American Revolution. These large guns were used mostly for defending forts--they weren't light field cannon.
But British, French, and Spanish ships-of-the-line (among others, surely) carried even larger guns, such as thirty-two pounders. Aside from the incredible danger of firing such a thing even with just powder (no shot), I can't even imagine the amount of damage such a beast could inflict. And to think of a gun deck--made of wood, naturally--carrying a dozen or more of these and bearing the weight of the gun and its ammunition, and bearing the force of their recoil, it boggles the mind.