Definition
The ratio of useful output produced by a machine, system, or process to the total input supplied to it. Efficiency is usually expressed as a percentage and is always less than 100% because some input energy is lost to friction, heat, or other losses.
Plain English
How much of what you put in actually does the job you wanted, compared to how much was wasted. Higher efficiency means less waste.
Context Anchor
Pilots encounter efficiency in discussions of aircraft performance, fuel use, propellers, engines, and aerodynamic design.
Derivation
From the Latin efficere, meaning 'to accomplish' or 'to bring about.' The word originally described the ability to actually get something done. In engineering, it now measures how well that 'getting it done' compares to the energy spent doing it.
Why Pilots Care
Higher efficiency directly improves range, reduces fuel costs, and allows safer operation on marginal reserves.
Intuition Check
Efficiency does not simply mean “fast” or “cheap to operate.” It means the useful result compared with what was put in to get that result.
Example Sentence 1
A propeller's efficiency drops sharply when the blade tips approach the speed of sound.
Example Sentence 2
At high altitude the propeller efficiency dropped because thinner air reduced the blade's ability to convert engine power into thrust.