In modern military terminology, a frigate is a warship intended to protect other warships and merchant ships as anti-submarine warfare (ASW) combatants for amphibious expeditionary forces, underway replenishment groups, and merchant convoys.

Sailing frigate and its rigging.

During the 18th and early 19th centuries, a frigate was a sailing vessel designed for speed, with a flush gun deck carrying 24 to 44 guns, used as a commerce raider and for blockade duty. With the introduction of steam and steel warships frigates as a class of warship passed out of use until World War II when they were reintroduced by the British as an antisubmarine escort vessel larger than a corvette but smaller than a destroyer.

Frigates in the United States Navy

The oldest commissioned warship in the US Navy is USF Constitution, better known as "Old Ironsides," a frigate put into service in the 1790s. It is the oldest commissioned warship afloat in the world; the HMS Victory, though older, is maintained in drydock.

In the United States Navy, guided missile frigates (with the FFG hull classification symbol) bring an anti-air warfare (AAW) capability to the frigate mission, but they have some limitations. Designed as cost-efficient surface combatants, they lack the multi-mission capability necessary for modern surface combatants faced with multiple, high-technology threats and offer limited capacity for growth.

In an attempt to overcome these limits and provide escorts that could keep up with nuclear-powered aircraft carriers, the US Navy commissioned several nuclear-powered frigates in the 1960s. They were far larger than any other frigates ever seen, and all were reclassified as cruisers in 1975 and struck from the Naval Vessel Register in the 1990s.

• USS Long Beach (DLGN/CGN-9) (Struck 1 January 1994)
• USS Bainbridge (DLGN/CGN-25) (Struck 6 October 1995)
• USS Truxtun (DLGN/CGN-35) (Struck 11 September 1995)
• USS California (DLGN/CGN-36) (Struck)
• USS South Carolina (DLGN/CGN-37) (Struck)
• USS Virginia (DLGN/CGN-38) (Struck 1 January 1994)
• USS Texas (DLGN/CGN-39) (Struck 16 July 1993)
• USS Mississippi (DLGN/CGN-40) (Struck 1995)
• USS Arkansas (DLGN/CGN-41) (Struck 1 October 1997)

All currently-operating frigates in the US Navy are conventionally-powered Oliver Hazard Perry-class ships.

The US Navy no longer has any need for single-mission ships and there are no frigates planned for the US Navy's future.

Partially from: http://www.chinfo.navy.mil/navpalib/factfile/ships/ship-ffg.html