Specific impulse
Change in velocity per amount of fuel / From Wikipedia, the free encyclopedia
Specific impulse (usually abbreviated Isp) is a measure of how efficiently a reaction mass engine, such as a rocket using propellant or a jet engine using fuel, generates thrust.
A propulsion system with a higher specific impulse uses the mass of the propellant more efficiently. In the case of a rocket, this means less propellant needed for a given delta-v,[1][2] so that the vehicle attached to the engine can more efficiently gain altitude and velocity.