An Ion Drive is the most common type of normal-space starship reaction drive. Ion drives use monatomic hydrogen as reaction mass, ionizing the hydrogen (stripping the electrons from the protons), and then electromagnetically ejecting the ions at a substantial fraction of the speed of light through the engine exhaust to provide thrust. This type of drive is incredibly mass-efficient and energy-efficient, but the total acceleration is still somewhat limited. Most starships can only pull a few G’s of acceleration, slightly less than a starfighter (but, of course, can sustain that acceleration a LOT longer than a fighter can).
Despite their impressive speed, it would still take years for an ion drive ship to cross between star systems. Therefore, ion drives are used mostly by commerical spacecraft to move between jumpgate along hyperspace shipping lanes, between a jumpgate and the planets of a solar system in normal space, or by warships to patrol, pursue, and maneuver during space battles.
Ion drives were invented in the early 2000's by David Farrell, and incrementally improved since then. Although slower in both accelleration and top speed than a Gravity Drive, they are much more compact and can be fit into smaller vessels, or allow more room in a larger vessel for things like weapons, armor, and fuel.