Definition of efficacy noun from the Oxford Advanced Learner's Dictionary

 

efficacy

 noun
noun
BrE BrE//ˈefɪkəsi//
 
; NAmE NAmE//ˈefɪkəsi//
 
[uncountable](formal)
 
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the ability of something to produce the results that are wanted synonym effectiveness to evaluate the efficacy and safety of the treatment Word Origin early 16th cent.: from Latin efficacia, from efficax, efficac-, from efficere ‘accomplish’, from ex- ‘out, thoroughly’ + facere ‘do, make’.
See the Oxford Advanced American Dictionary entry: efficacy