Energy conservation, reducing the use of energy

Energy efficiency, useful work per quantity of energy

Efficiency wages, paying workers more than the market rate for increased productivity

Fuel efficiency, converting energy in fuel to kinetic energy or work

Allocative efficiency, an optimal distribution of goods

Specific impulse, in propulsion systems, a measure of fuel-efficiency

The optimisation of a social welfare function

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

Efficiency (aerodynamics), for fixed wing aircraft the Lift-to-drag ratio

Kaldor-Hicks efficiency, like a less stringent Pareto efficiency

Thermodynamic efficiency, useful work per change in heat

Thermal efficiency, useful work per the higher heating value of the fuel

Efficiency Movement, of the Progressive Era 1890-1932, advocated efficiency in the economy, society and government

Exergy efficiency, measure of 2nd-law themodynamic loss

Energy conversion efficiency, desired energy output per energy input

Efficiency (statistics), a measure of desirability of an estimator

The term inefficiency has several meanings depending on the context in which its...

Electrical efficiency, useful power output per electrical power consumed

Algorithmic efficiency, the speed and space requirements of a computer program

Volumetric efficiency, in internal combustion engine design

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

Efficient (horse), winner of the 2007 Melbourne Cup.

Efficiency ratio, expenses as a percentage of revenue, etc.

Material efficiency, compares material requirements between construction projects or physical processes

Mechanical efficiency, the effectiveness of a simple machine, mechanical advantage over ideal mechanical advantage