Isp - the measure of efficiency of rocket (does not apply to reactionless drive); how much impulse (thrust multiplied by time) is produced per unit mass of propellant.
Specific Impulse is also equivalent to exhaust velocity, but by convention is measured in units per second (which means dividing exhaust velocity by 1g (9.81 meters/sec.)
Chemical propellants and rockets are the most inefficient, in terms of Isp, although giving a very good thrust, and being free of harmful radioactive particles. Nuclear and Ion propulsion both have a higher Isp, but either produce insufficient thrust for planetary lift-off (nuclear fusion and ion) or create harmful radioactive articles (nuclear fission). Amat and conversion drive are more efficient again (reaching the maximum possible Isp) but produce an even more deadly radioactive exhaust. Hence high Isp rockets are only used in deep space, away from biont inhabited areas.