Specific impulse (commonly abbreviated Isp) is the impulse (change in momentum) per unit mass for rocket fuels, or rather how much more push accumulates as you use that fuel. The velocity of a rocket depends on thrust (which is roughly the mass of propellant that is thrown out of the back of the rocket and the velocity at which that propellant is thrown out) compared to the rocket's mass.
The faster the speed at which propellant is thrown out the back of the rocket, the faster the rocket can travel or the more cargo it can carry. The specific impulse of a rocket propellant is a rough measure of how fast the propellant is ejected out of the back of the rocket. A rocket with a high specific impulse doesn't need as much fuel as a rocket with low specific impulse to reach a given velocity.
The specific impulse as defined above is equal to the exhaust velocity ueq, which is the total impulse divided by the mass of the expelled propellant.
Alternatively the specific impulse is defined as the change in momentum per unit weight:
In this case Isp is measured in seconds.
The specific impulse for various means of propulsion are given in the entry for spacecraft propulsion.
