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Specific Impulse

The impulse produced by a rocket divided by the mass $mp$ of propellant consumed. Specific impulse ${I_{sp}}$ is a widely used measure of performance for chemical, nuclear, and electric rockets. It is usually given in seconds for both U.S. Customary and International System (SI) units. The impulse produced by a rocket is the thrust force $F$ times its duration $t$ in seconds. $I_{sp}$ is the thrust per unit mass flowrate, but with $g_o$, is the thrust per weight flowrate. The specific impulse is given by the equation: $I_{sp} = \frac{F}{\dot{m}g_o}$.

http://www.grc.nasa.gov/WWW/K-12/airplane/specimp.html