A ramjet, sometimes referred to as a stovepipe jet, or an athodyd, is a form of airbreathing jet engine

Airbreathing jet engine

An airbreathing jet engine is a jet engine propelled by a jet of hot exhaust gases formed from air that is drawn into the engine via an inlet duct....

 using the engine's forward motion to compress incoming air, without a rotary compressor. Ramjets cannot produce thrust at zero airspeed and thus cannot move an aircraft from a standstill. Ramjets therefore require some other propulsion system to accelerate the vehicle to a speed where the ramjet begins to produce thrust.