A measure o' t' efficiency o' a rocket engine. Similar t' Specific Impulse, it is defined as t' Total Impulse divided by t' mass o' t' propellants. A little dimensional jugglin' shows that this gives t' same units as velocity (ft/sec or m/sec) hence is sometimes called "Effective Exhaust Velocity." How smartly t' reaction mass leaves t' nozzle is a good measure o' efficiency.