In the 18th and 19th century, a sloop-of-war was a small sailing warship with a single gun deck which carried between ten and eighteen cannons. A brig sloop had two masts and a ship sloop had three, because a brig in those days was a one or two masted vessel - to be a ship it had to have three or more masts. A sloop-of-war was quite different from a civilian sloop, which was a general term for a single masted vessel.
Successive generations of guns became larger in the second half of the 19th century, so by the 1880s even the most powerful warships had less than a dozen large calibre guns. The term had by then become much less precise, meaning a small warship with a single gun-deck and which was bigger than a gunboat. By the second world war it had come to mean a small warship armed with one or two 4 inch guns and depth charges.
