b (1): effective operation as measured by a comparison of production with cost (as in energy, time, and money) (2): the ratio of the useful energy delivered by a dynamic system to the energy supplied to it
In mechanics, the measure of the effectiveness with which a system performs. It is stated as the ratio of a system's work output to its work input. The efficiency of a real system is always less than 1 because of friction between moving parts. A machine with an efficiency of 0.8 returns 80% of the work input as work output; the remaining 20% is used to overcome friction. In a theoretically frictionless, or ideal, machine, the work input and work output are equal, and the efficiency would be 1, or 100%.