Mitrailleuse (literally "grapeshot shooter") is the French word used to describe all rapid-firing weapons of rifle caliber.
Therefore the word "mitrailleuse", when used in the French language, applies to all machine guns including modern full automatic weapons. However in the English language the word "mitrailleuse" applies only to volley guns with multiple barrels of rifle caliber. The earliest true "mitrailleuse" was invented in 1851 by Belgian Army Captain Fafschamps, 10 years before the advent of the Gatling gun. It was followed by the Belgian Montigny mitrailleuse in 1863. Then the French 25 barrel "Canon à Balles", better known as the Reffye mitrailleuse, was adopted in great secrecy in 1866. It became the first rapid-firing weapon to be deployed as standard equipment by any army in a major conflict. This happened during the Franco-Prussian War of 1870-71.
Mitrailleuses were used by the navy of the United States of Europe, which mounted them on ironclads and timberclads. The naval mitrailleuses were based on the Reffye mitrailleuse, but used 20 barrels instead of 25. Unlike the army's volley guns, which were mounted as artillery, they were on pivot mounts, and could be traversed. Also, they were selective fire weapons; they were fired by turning a crank, and their rate of fire depended on how fast the crank was turned.