A reactionless drive is a hypothetical device producing motion without the exhaust of a propellant. A propellantless drive is not necessarily reactionless when it constitutes an open system interacting with external fields; but a reactionless drive is a particular case of a propellantless drive that is a closed system, presumably in contradiction with the law of conservation of momentum. Reactionless drives are often considered similar to a perpetual motion machine.[1] The name comes from Newton's third law, often expressed as: "For every action, there is an equal and opposite reaction."
Many infeasible reactionless drives are a staple of science fiction for space propulsion.