efficient

Etymology

1398, “making,” from Old French, from Latin efficientem, nominative efficiēns, participle of efficere (“work out, accomplish”) (see effect). Meaning “productive, skilled” is from 1787. Efficiency apartment is first recorded 1930, American English.

adj

1. making good, thorough, or careful use of resources; not consuming extra. Especially, making good use of time or energy
   An efficient process would automate all the routine work.
   Our cleaners are almost too efficient: they throw away anything left out on a desk.
   An artificial kidney these days still means a refrigerator-sized dialysis machine. Such devices mimic[…]real kidneys[…]. But they are nothing like as efficient, and can cause bleeding, clotting and infection—not to mention inconvenience for patients, who typically need to be hooked up to one three times a week for hours at a time. 2013-06-01, “A better waterworks”, in The Economist, volume 407, number 8838, page 5 (Technology Quarterly)
2. expressing the proportion of consumed energy that was successfully used in a process; the ratio of useful output to total input
   The motor is only 20% efficient at that temperature.
3. causing effects, producing results; bringing into being; initiating change (rare except in philosophical and legal expression efficient cause = causative factor or agent)
   Ownership, maintenance, or use of the automobile need not be the direct and efficient cause of the injury sustained
4. (proscribed, old use) effective, efficacious
   The Efficient Cause is that from which emanates the force that produces the Effect 1856, William Dexter Wilson, An Elementary Treatise on Logic

noun

1. (obsolete) a cause; something that causes an effect
   Some are without efficient, as God; others without matter, as Angels […]. 1643, Thomas Browne, Religio Medici, I.14
   This implies, that something happens without a cause. If it should be said, that motive in this case is not the efficient of the action or doing — this is granted; but at the same time, for reasons already given, it is denied, that the man himself is the efficient cause of it. a. 1758, Jonathan Edwards, Dissertation on Liberty and Necessity