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.
British Dictionary definitions for specific impulse
specific impulse
noun
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
