Reactionless drive
Reactionless drive refers to hypothetical and unproven forms of spacecraft propulsion that would generate thrust without the expelling propellant or otherwise exchanging momentum. A propellantless drive is not necessarily reactionless if it functions as an open system interacting with external fields, but a reactionless drive is generally conceived as a self-contained or closed-system device, which is why such claims are commonly treated as conflicting with Newton's third law and the conservation of momentum.