A fleet is usually a large group of ships, but it can be any group of vessels like planes or cars that operate as a unit. A naval fleet is the largest formation of warships. A naval fleet at sea is like an army on land.
One meaning of fleet comes from the Old English fleot, meaning “ship,” “raft,” or “floating vessel.” Although a fleet is usually a group of ships or airplanes operating under the same ownership, any group of vessels sailing together can be called a fleet, even you and your friends in a fleet of kayaks. Another meaning of fleet originates in the Old English flēotan, which means “moving swiftly.” When it’s used in that way, it’s usually in the -ing form, fleeting.