Specific impulse (Isp) is a measure of how efficiently a rocket engine uses its propellant. It is defined as the thrust produced per unit of propellant mass flow rate. In simpler terms, it's like "miles per gallon" for a rocket.

A higher specific impulse means the engine is more efficient, requiring less fuel to achieve the same change in velocity (delta-v). Chemical rockets usually have Isp values between 250 and 450 seconds, while ion thrusters can reach several thousand seconds.