Efficient n. : Causing effects; producing results; that makes the effect to be what it is; actively operative; not inactive, slack, or incapable; characterized by energetic and useful activity; as, an efficient officer, power.

Efficient n. : An efficient cause; a prime mover.

Example:
How Efficient are power inverters that plug into the cigarette lighter?

Taxonomy
Dictionary, ef, Synonyms, fi, nt, adjective, word origin, Acting, Houghton Mifflin Company, free online English dictionary, dictionary results, pronunciation, waste of time, free online dictionary, definitions, websters, spelling, vocabulary.

Defination from the web:
Efficient - Definition of Efficient at Dictionary.com a free online dictionary with pronunciation, synonyms, and translation of Efficient. ... least waste of time and effort; having and using requisite knowledge, skill, and industry; competent; capable: a reliable, efficient secretary. ...