- a measure, usually in seconds, of the efficiency with which a rocket engine utilizes its propellants, equal to the number of pounds of thrust produced per pound of propellant burned per second.
- the specific impulse that a given combination of propellants would produce in an ideal rocket engine providing complete combustion, no friction losses, and no lateral expansion of the exhaust.
- the ratio of the thrust produced by a rocket engine to the rate of fuel consumption: it has units of time and is the length of time that unit weight of propellant would last if used to produce one unit of thrust continuously
