Q-drive

The Q-drive, also known as propellant tacking, plasmadyne jet or plasma clipper, is a reaction engine driven by the dynamic pressure $q$ of a space plasma wind (such as the interstellar medium as a spaceship moves through it). Power is extracted from the flowing plasma and used to accelerate reaction mass stored aboard the vehicle.

((STUB to get the link structure to work; will add shortly -- rocketguy ))