The Oxford Thesaurus An A-Z Dictionary of Synonyms - efficiencyn.

1) effectiveness, efficacy, competence, capability, adeptness, proficiency, expertness, expertise, know-how, experience, skill, skilfulness, dexterity, adroitness: The efficiency of the staff has been greatly improved. 2) productivity, effectiveness, efficaciousness: How does the efficiency of a diesel compare with that of other engines?