What is "sloop-of-war"

In the 18th century and most of the 19th, a sloop-of-war in the Royal Navy was a warship with a single gun deck that carried up to eighteen guns. The rating system covered all vessels with 20 guns and above; thus, the term sloop-of-war actually encompassed all the unrated combat vessels, including the very small gun- brigs and cutters. In technical terms, even the more specialised bomb vessels and fireships were classed as sloops-of-war, and in practice these were actually employed in the sloop role when not carrying out their specialized functions.

In World War I and World War II, the Royal Navy reused the term "sloop" for specialized convoy-defence vessels, including the of World War I and the highly successful of World War II, with anti-aircraft and anti-submarine capability.