比冲或比冲量是对一个推进系统的燃烧效率的描述。比冲的定义为：火箭发动机单位重量推进剂产生的冲量，或单位重量（重力）流量的推进剂产生的推力。比冲的单位为米/秒（m/s）或牛·秒/千克（N·s/kg）（考虑单位质量流量的推进剂），工程上会考虑单位重量流量的推进剂，而比冲的单位习惯使用秒（s）。比冲越高代表效率越好，亦即可以用相同质量的燃料产生更多的动量。

Specific impulse (usually abbreviated I_sp) is a measure of the efficiency of rocket and jet engines. By definition, it is the total impulse (or change in momentum) delivered per unit of propellant consumed, and is dimensionally equivalent to the thrust generated divided by the propellant flow rate. If mass (kilogram or slug) is used as the unit of propellant, then specific impulse has units of velocity. If weight (newton or pound) is used instead, then specific impulse has units of time (seconds). Multiplying flow rate by the gravitational constant (g₀) before dividing it into the thrust, converts specific impulse from the mass basis to the weight basis.