A ship propulsion system similar in nature to Anti-Gravity. It is also called the Gravity Drive.
A gravity drive works by generating gravitational waves to push the vessel forward. Effectively, the ship 'falls' forward and with no atmosphere to slow it down, ships can reach incredible speeds very fast. A graviton drive is the one system with the best chance of literally approaching light speed in reasonable time.
Most ships only have a single Gravity Drive Engine installed on board due to the extreme thrust they can create. They are also disrupted by natural gravity wells which why they originally faded out of use. Some version were upgraded to take gravity wells into consideration.
Gravity Drives are the direct predecessors of the Warp Drive.