An unmanned aerial vehicle (UAV; also known as a remotely piloted vehicle or RPV, or Unmanned Aircraft System (UAS)) is an aircraft that is flown by a pilot; however, without a human crew on board the aircraft. Their largest uses are in military applications. To distinguish UAVs from missiles, a UAV is defined[by whom?] as a reusable, remotely crewed aircraft capable of controlled, sustained, level flight and powered by a jet, reciprocating engine, or other source of propulsion. Therefore, cruise missiles are not considered UAVs, because, like many other guided missiles, the vehicle itself is a weapon that is not reused, even though it is also unmanned and in some cases remotely guided.

There are a wide variety of UAV shapes, sizes, configurations, and characteristics. Historically, UAVs were simple drones[1] (remotely piloted aircraft), but autonomous control is increasingly being employed in UAVs. UAVs come in two varieties: some are controlled from a remote location, and others fly autonomously based on pre-programmed flight plans using more complex dynamic automation systems.

Currently, military UAVs perform reconnaissance as well as attack missions.[2] While many successful drone attacks on militants have been reported, they are also prone to collateral damage and/or erroneous targeting, as with many other weapon types.[1] UAVs are also used in a small but growing number of civil applications, such as firefighting or nonmilitary security work, such as surveillance of pipelines. UAVs are often preferred for missions that are too "dull, dirty, or dangerous" for manned aircraft.

The abbreviation UAV has been expanded in some cases to UAVS (unmanned-aircraft vehicle system). In the United States, the Federal Aviation Administration has adopted the generic class unmanned aircraft system (UAS) originally introduced by the U.S. Navy to reflect the fact that these are not just aircraft, but systems, including ground stations and other elements.