<P> In the 18th century, the term referred to ships that were usually as long as a ship of the line and were square - rigged on all three masts (full - rigged), but were faster and with lighter armament, used for patrolling and escort . In the definition adopted by the British Admiralty, they were rated ships of at least 28 guns, carrying their principal armaments upon a single continuous deck--the upper deck--while ships of the line possessed two or more continuous decks bearing batteries of guns . </P> <P> In the late 19th century (beginning about 1858 with the construction of prototypes by the British and French navies), the armoured frigate was a type of ironclad warship that for a time was the most powerful type of vessel afloat . The term "frigate" was used because such ships still mounted their principal armaments on a single continuous upper deck . </P> <P> In modern navies, frigates are used to protect other warships and merchant - marine ships, especially as anti-submarine warfare (ASW) combatants for amphibious expeditionary forces, underway replenishment groups, and merchant convoys . Ship classes dubbed "frigates" have also more closely resembled corvettes, destroyers, cruisers, and even battleships . Some European navies such as the French, German or Spanish ones use the term "frigate" for both their destroyers and frigates . The rank "frigate captain" derives from the name of this type of ship . </P> <P> The term "frigate" (Italian: fregata; Spanish / Catalan / Portuguese / Sicilian: fragata; Dutch: fregat; French: frégate) originated in the Mediterranean in the late 15th century, referring to a lighter galleass type ship with oars, sails and a light armament, built for speed and maneuverability . The etymology of the word is unknown, although it may have originated as a corruption of aphractus, a Latin word for an open vessel with no lower deck . Aphractus was, in turn, derived from the Ancient Greek phrase ἄφρακτος ναῦς (aphraktos naus), or "undefended ship". </P>

What is the role of a navy frigate