1. A motion produced in reaction to the expulsion of a stream of accelerated ions.
2. Vehicular motion caused by reaction from the high-speed discharge of a beam of electrically charged minute particles, usually positive ions, that are accelerated in an electrostatic field and ejected behind the vehicle.
3. A propulsor (mechanical device that gives propulsion), usually a small thruster, used to create vehicular motion by generating a high-velocity jet of ions in an electrostatic field, then ejecting the ions behind a vehicle.
4. Propulsion by the reactive thrust of a high-speed beam of similarly charged ions ejected by an ion engine.