A measure of the overall effectiveness of a system. For example, in power conversion, it would be the ratio of output power to input power.
Efficiency is the fraction of a quantity that can be used to perform a desired task. For example, in power conversion and transmisison systems it would be the fraction of the input power that reaches the output load. Typically, the more efficient a system is, the better.
The term can also be applied in other ways. In information processing and communications, efficiency can refer to the fraction of physical bits or symbols that actually convey application data, as opposed to "overhead" bits used for synchronization or error detection/correction.
