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 assistant.
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 the least waste of resources or effort (usually used in combination): a fuel-efficient engine.
functioning or producing effectively and with the least waste of effort; competent