Definitions for "Specific impulse"
A measurement for determining the relative performances of a rocket propellant that demonstrates how efficient the rocket is. Specific impulse is basically equivalent to exhaust velocity, it is actually exhaust velocity divided by the constant g. As a result, specific impulse has units of seconds. Physically, it is the number of seconds a rocket engine can produce one pound of thrust from one pound of fuel, so a specific impulse of 1,000,000 means that the engine gives 1,000,000 pounds of thrust, for one second, for every pound of fuel consumed. The higher the specific impulse a rocket has, the less fuel it requires to produce a given amount of thrust.
Measure of fuel efficiency.
The force per unit mass per unit time generated by a rocket propellant under fixed conditions of pressure, temperature, and mixture ratio.