From Envirowiki

Efficiency is a measure of output as a percentage of input. For example, energy efficiency is a measure of the efficiency of the energy conversion process: a solar panel is 30% efficient if it emits 30% of the energy in electricity as it receives in light. Material efficiency is a measure of waste - the less efficiency, the more waste. Physical efficiency cannot exceed 100% (otherwise we wouldn't have any problems).

Efficiency is highly valued in capitalist society, especially economic efficiency, as it increases profit. However, the benefits of efficiency (that it reduces input), may have negative side effects, including less meaningful work, leading to poverty, and also possibly increasing resource use over all by making the resource easier to use (Jevon's paradox).