The EMDrive is a type of electromagnetic thruster that is being researched. Unlike conventional electric thrusters, it does not use any form of reaction mass.
RF resonant cavity thrusters are a type of electromagnetic thruster that is being researched. Unlike conventional electric thrusters, they do not use any form of reaction mass, nor are they designed to emit any directional radiation.
A few designs for such thrusters have been proposed. aerospace engineer Roger Shawyer designed the EmDrive in 2001, and has persistently promoted the idea since then through his company, Satellite Propulsion Research. Chemical engineer Guido Fetta designed the Cannae Drive, operating on similar principles.
A few independent teams of physicists, notably a team at Xi'an's Northwestern Polytechnical University (NWPU) and one at NASA's Eagleworks Laboratories, have built prototypes of these designs, and have tentatively observed a small amount of net thrust from those prototypes. None of this experimental work has been published in independent peer reviewed journals, in part because observed thrust has been small, there are no compelling explanations for the apparent violation of momentum conservation, and claims of such a surprising result would need especially thorough efforts to rule out possible sources of error.