Efficiency could mean:
Energy efficiency, useful work per quantity of energy
Energy conversion efficiency, desired energy output per energy input
Energy conservation, reducing the use of energy
Electrical efficiency, useful power output per electrical power consumed
Fuel efficiency, converting energy in fuel to kinetic energy or work
Specific impulse, in propulsion systems, a measure of fuel-efficiency
Volumetric efficiency, in internal combustion engine design
Lighting efficiency, visible light from a light source per unit of power, in lumens per watt Mechanical efficiency, the effectiveness of a simple machine, mechanical advantage over ideal mechanical advantage Thermodynamic efficiency, useful work per change in heat Exergy efficiency, measure of 2nd-law themodynamic loss Thermal efficiency, useful work per the higher heating value of the fuel Radiation efficiency, ratio of radiated power to power absorbed at the terminals of an antenna Efficiency (aerodynamics), for fixed wing aircraft the Lift-to-drag ratio Efficiency (economics), a general term, to capture the amount of waste or other undesirable features Pareto efficiency, making one individual better off, without making any other individual worse off Kaldor-Hicks efficiency, like a less stringent Pareto efficiency Allocative efficiency, an optimal distribution of goods The optimisation of a social welfare function Efficiency wages, paying workers more than the market rate for increased productivity Efficiency (statistics), a measure of desirability of an estimator Efficiency ratio, expenses as a percentage of revenue, etc. Efficiency Movement, of the Progressive Era 1890-1932, advocated efficiency in the economy, society and government Material efficiency, compares material requirements between construction projects or physical processes Textual efficiency, density of a message Algorithmic efficiency, the speed and space requirements of a computer program Quantum efficiency, a measure of sensitivity of a photosensitive device: the percentage of photons hitting the photoreactive surface that will produce an electron–hole pair