English · Word Candidate

efficacy

/ˈɛf.ɪ.kə.si/

Proposed by a contributor, not yet supported by evidence.

Also seen as: efficacies

Sense 1

Meaning · English

The ability to produce a desired effect under ideal testing conditions.

Noun

Examples

[…]and this hath even made me ſuſpect the efficacy of reliques, to examine the bones, queſtion the habits and appurtenances of Saints, and even of Chriſt himſelf.

Usage example from English Wiktionary (CC BY-SA 4.0, retrieved 2026-08-01)

Sources & notes

Adapted from English Wiktionary (CC BY-SA 4.0, retrieved 2026-08-01 via kaikki.org).

Sense 2

Meaning · English

A degree of ability to produce a desired effect; effectiveness.

Noun

Examples

Toothbrush with improved efficacy

Usage example from English Wiktionary (CC BY-SA 4.0, retrieved 2026-08-01)

Sources & notes

Adapted from English Wiktionary (CC BY-SA 4.0, retrieved 2026-08-01 via kaikki.org).

Common questions

What does “efficacy” mean?

The ability to produce a desired effect under ideal testing conditions.

Is “efficacy” formal or informal?

No register label has been reviewed yet — treat it as neutral and check the usage notes above.

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