In historical naval

Navy

A navy is the branch of a nation's armed forces principally designated for naval and amphibious warfare; namely, lake- or ocean-borne combat operations and related functions...

 usage, a long gun was the standard type of cannon

Cannon

A cannon is any piece of artillery that uses gunpowder or other usually explosive-based propellents to launch a projectile. Cannon vary in caliber, range, mobility, rate of fire, angle of fire, and firepower; different forms of cannon combine and balance these attributes in varying degrees,...

 mounted by a sailing vessel, so called to distinguish it from the much shorter carronade

Carronade

The carronade was a short smoothbore, cast iron cannon, developed for the Royal Navy by the Carron Company, an ironworks in Falkirk, Scotland, UK. It was used from the 1770s to the 1850s. Its main function was to serve as a powerful, short-range anti-ship and anti-crew weapon...

s.

In informal usage, the length was combined with the weight of shot, yielding terms like "long nines", referring to full-length, 9-pounder guns. They were mostly used as bow and stern chasers

Chase gun

The chase guns, usually distinguished as bow chasers and stern chasers were cannons mounted in the bow or stern of a sailing ship...

because their length was too much for the width of the ship.