Other forms: efficiencies
Efficiency is getting the result you want without wasting time, money or effort on the way. A dishwasher that cleans a full load on two litres of water, a route to work that saves ten minutes every morning, a kitchen where nobody has to walk twice.
The adjective is efficient and the adverb efficiently: an efficient boiler, a meeting run efficiently. In physics and engineering, efficiency is an exact number — useful energy out divided by energy in, written as a percentage, and never above a hundred. Business English keeps the word close to money: fuel efficiency, energy efficiency, and 'efficiency savings', which is the polite way a company announces job cuts. Efficiency is not the same as effectiveness — efficiency is doing the job with little waste, effectiveness is doing the job that mattered. In American property listings an efficiency is something else again: a small flat with the cooking, sitting and sleeping all in one room.
1nounefficiencyThe quality of getting a result with little waste of time, money, effort or material.
“The new sorting system doubled the efficiency of the warehouse.”
synonyms: productivity, economy
2nounusefulThe useful work or energy a machine gives out as a share of what it takes in, given as a percentage.
“The panels reach an efficiency of about twenty per cent in full sun.”
3nounsmallIn American English, a small flat with kitchen, living and sleeping space in a single room.
“She rented an efficiency two streets from the hospital.”
efficiency is a quality.
English took efficiency from Latin efficientia, and the plainer route shows in the word itself: efficient plus the ending -cy, which turns a quality into something you can name and measure. That is the whole job of the word. It takes 'this works well' and makes it a thing you can put a number on.
Latin
efficientia
English, today
efficiency
From Latin efficientia, equivalent to efficient + -cy.
Etymology from Wiktionary, CC BY-SA