efficacy Meaning, Definition & Usage

  1. noun capacity or power to produce a desired effect
    efficaciousness.
    • concern about the safety and efficacy of the vaccine

WordNet

Ef"fi*ca*cy noun
Etymology
L. efficacia, fr. efficax. See Efficacious.
Definitions
  1. 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. "Of noxious efficacy." Milton. Syn. -- Virtue; force; energy; potency; efficiency.

Webster 1913