Plasma cannons are a form of weaponry. On Starfleet vessels, they were the precursors to phase cannons. Cargo vessels were commonly equipped with one turret, designed to break up asteroids and meteors.
The Enterprise was equipped with several plasma cannons during the first days of its mission, but was eventually upgraded with the more advanced phase cannons. (ENT: "Broken Bow") Enterprise's Shuttlepod 1 was equipped with dual plasma cannons. (ENT: "Awakening") These plasma cannons had a range of less than ten kilometers. (ENT: "Shuttlepod One")
Earth Y class freighters, were typically equipped with a single plasma cannon. The captains of these freighters, however, would often upgrade them at the first chance they got. Often referred to as "popguns", to achieve a weapons lock, the target had to be within a nine kilometer range. (ENT: "Fortunate Son")
In an alternate timeline, Vosk and the Na'kuhl offered Nazi Germany plasma cannons and other weapons in exchange for materials to build the temporal conduit. (ENT: "Storm Front, Part II")
In the Mirror universe in 2155 the Terran Empire still used plasma cannons on NX battleships. (ENT: "In a Mirror, Darkly", "In a Mirror, Darkly, Part II")
