# efficacy

> English word · Noun · IPA /ˈɛf.ɪ.kə.si/ · frequency rank #11,537

## Definitions
1. The ability to produce a desired effect under ideal testing conditions.
2. A degree of ability to produce a desired effect; effectiveness.

## Etymology
From Old French efficace, from Late Latin efficācia (“efficacy”), from efficāx (“efficacious”); see efficacious.

## Common misspellings (10)
`effciacy`, `effiaccy`, `efficaccy`, `efficacyy`, `efficayc`, `efficcacy`, `efficcay`, `eficacy`, `efifcacy`, `feficacy`

## Source
Compiled from Wiktionary via kaikki.org (CC BY-SA). Data vintage: 2026-05-06.
Canonical page: https://plainspell.com/en/word/efficacy