1398, "making," from Old French, from Latin efficientem, nominative efficiens, participle of efficere (“work out, accomplish”) (see effect). Meaning "productive, skilled" is from 1787. Efficiency apartment is first recorded 1930, American English. 
- Making good, thorough, or careful use of resources; not consuming extra. Especially, making good use of time or energy.
2013 June 1, “A better waterworks”, in The Economist, volume 407, number 8838, page 5 (Technology Quarterly):
- 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.
An efficient process would automate all the routine work.
Our cleaners are almost too efficient: they throw away anything left out on a desk.
- 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.
- 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
- (proscribed, old use) Effective.
- The efficient cause is the working cause.
Webster's Revised Unabridged Dictionary from 1913 still lists efficient and effective as synonyms, but all major dictionaries now show that these words now only have different meanings in careful use. Use of both for the other meaning is however widespread enough that Longman's Exam Dictionary, for example, finds it necessary to proscribe the use of one for the other with several examples at each entry and provides the following summary:
- efficient = working quickly and without waste
- effective = having the desired effect