Noun
a sailing or steam warship having cannons on only one deck
Source: WordNetsloop-of-war
John Paul Jones became notorious in Britain for his expedition from France in the little sloop of war Ranger in April 1778, during which, in addition to his attacks on merchant shipping, he made two landings on British soil. Source: Internet
Note that a " sloop of war ", in the naval sense, may well have more than one mast, and is not properly a sloop by the modern meaning. Source: Internet