Making good, thorough, or careful use of resources; not consuming extra. Especially, making good use of time or energy. (adjective)
Using a particular proportion of available energy. (adjective)
Examples of word efficient
If he left the "road" and became a _very_ efficient common laborer, some _ordinarily efficient_ common laborer would have to take to the "road."
Now, when the advocates of free-agency insist that motive is not the efficient cause of volition, and that mind is the efficient cause thereof, we suppose them to employ the expression, _efficient cause_, in one and the same sense in both branches of the proposition.
 I use the term efficient in a technical sense, as meaning all-sufficient to produce the given effect, without the intervention of any other cause.
That isn't to deny that a war with tens of thousands of people doesn't have a significant administrative overhead; rather, that focusing on this overhead (which laughs at the term efficient) ends up seriously degrading our ability to actually achieve victory.
Sure, the critique depends on misinterpreting what the word "efficient" means, as in the "efficient markets hypothesis."