performing or functioning in the best possible manner with the least waste of time and effort; having and using requisite knowledge, skill, and industry; competent; capable

a reliable, efficient secretary.

satisfactory and economical to use

Our new air conditioner is more efficient than our old one.

producing an effect, as a cause; causative.

utilizing a particular commodity or product with maximum efficiency (usually used in combination)

a fuel-efficient engine.

adjective

helpless , impotent , incompetent , ineffective , inefficient , powerless , unable , weak

adjective

able , accomplished , active , adapted , adequate , apt , businesslike , capable , clever , competent , conducive , decisive , deft , dynamic , economic , economical , effectual , efficacious , energetic , equal to , experienced , expert , familiar with , fitted , good at , good for , handy , masterly , organized , potent , powerful , practiced , productive , proficient , profitable , qualified , ready , saving , shrewd , skilled , skillful , systematic , talented , tough , useful , valuable , virtuous , well-organized , adept , consummate , dexterous , effective , expeditious , ingenious , professional , virtuosic