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Added 9 October 2026

efficacy

/ˈɛf.ɪ.kə.si/ noun

In Latin: efficacitas Wiktionary

noun Definition from WordNet 3.1, Princeton University. Pronunciation from Wiktionary, via Kaikki.org. More about efficacy at Wordnik → Where our words come from →

Capacity or power to produce a desired effect.
concern about the safety and efficacy of the vaccine

Origin

From Old French efficace, from Late Latin efficācia (“efficacy”) (whence -acy), from efficāx (“efficacious”); see efficacious.

From Wiktionary, via Kaikki.org (CC BY-SA)

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