Efficacy

n. effectiveness, ability to produce the desired result

Efficacy is the ability to produce a desired amount of a desired effect.
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Noun
1. capacity or power to produce a desired effect; "concern about the safety and efficacy of the vaccine"
(synonym) efficaciousness
(antonym) inefficacy, inefficaciousness
(hypernym) effectiveness, effectivity, effectualness, effectuality
(attribute) efficacious

(n.)
Power to produce effects; operation or energy of an agent or force; production of the effect intended; as, the efficacy of medicine in counteracting disease; the efficacy of prayer.
  

Effectiveness. In medicine, the ability of an intervention (for example, a drug or surgery) to produce the desired beneficial effect.