Webster
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Webster’s Revised Unabridged Dictionary · 1913

Efficacy

Ef′fi·ca·cy (ĕf′fĭ·kȧ·sy̆), n. [L. efficacia, fr. efficax. See Efficacious.] 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’s Revised Unabridged Dictionary, Noah Porter, editor (Springfield, Mass.: G. & C. Merriam, 1913). Public domain. Text as transcribed by MICRA, Inc., from Project Gutenberg. Changed for Deep Dive Bible: Greek, which our source spelled in Latin letters, set in Greek letters; entries of one spelling merged into one article; Greek words our source left out (shown as “?”), read from the 1898 printing, from the same plates (the Library of Congress’s copy), where the place could be found with certainty. Of the others, 1,362 (a word or a single letter each) were inferred by AI from the context by the dictionary’s own rules and are marked with a wavy underline (tap or point at one to see the note); the rest, which could not be inferred with confidence, are left as “?”.