adjective
Making good, thorough, or careful use of resources; not consuming extra. Especially, making good use of time or energy.
Expressing the proportion of consumed energy that was successfully used in a process; the ratio of useful output to total input.
Causing effects, producing results; bringing into being; initiating change (rare except in philosophical and legal expression efficient cause = causative factor or agent).
(proscribed, old use) effective, efficacious
noun
(obsolete) A cause; something that causes an effect.