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Specific impulse (usually abbreviated I_sp) is a way to describe the efficiency of rocket and jet engines. It represents the impulse (change in momentum) per unit of propellant. The higher the specific impulse, the less propellant is needed to gain a given amount of momentum. I_sp is a useful value to compare engines, much like "miles per gallon" or "litres per kilometre" is used for cars. A propulsion method with a higher specific impulse is more propellant-efficient.

Propellant is normally measured either in units of mass, or in units of weight at sea level on Earth. If mass is used, specific impulse is an impulse per unit mass, which dimensional analysis shows to be a unit of speed, and so specific impulses are often measured in metres per second, and are often termed effective exhaust velocity. However, if propellant weight is used instead, an impulse divided by a force (weight) turns out to be a unit of time, and so specific impulses are measured in seconds. These two formulations are both widely used, and differ from each other by a factor of g, the dimensioned constant of gravitational acceleration at the surface of the Earth.

Another number that measures the same thing, usually used for air-breathing jet engines, is Specific fuel consumption. Specific fuel consumption is inversely proportional to specific impulse and effective exhaust velocity.