efficacy

/ˈɛf.ɪ.kə.si/

//ˈɛf.ɪ.kə.si// noun

By · According to Wiktionary data (CC BY-SA) ·

"efficacy" is a 8-letter English headword indexed on PlainSpell.

The verdict

“efficacy” has 10 generated spelling variants in the English index at frequency #11,537. The variants make it a useful spelling check.

#11,537
frequency rank, English
2,246
“E” headwords
10
tracked misspellings

According to Wiktionary data (CC BY-SA, analyzed September 3, 2026) - The ability to produce a desired effect under ideal testing conditions.

Corpus desk

Index EN-efficacy · efficacy · English

efficacy · rank #11,537 · 10 variants · 0 confusables

  • FREQ-MID #11,537
  • LEN-LONG 8 letters
  • VOW-4 4 vowels
  • CONFUS-0 No pairs
  • VAR-MID 10 variants
  • BOOK-MID 2,246
  • PHOTO-FINISH durable

Nearest frequency peer: durable (-1 rank slots)

Read with this headword: Spelling guide for this pattern

Desk codes from Wiktionary headword shape, FrequencyWords rank tier, confusable-pair count, generated misspelling depth, letter-cohort book size, and photo-finish peer. Not a difficulty grade.

Rank peers near misspelling magnet “efficacy”

Same rarity band: useful when a typo lands on a peer headword

corpus weight

What this shows “efficacy” sits among these FrequencyWords peers; misspelling depth is tracked separately in the edit-distance chart.

Source FrequencyWords open word-frequency list As of September 3, 2026
Key facts for efficacy
PropertyValue
Headwordefficacy
LanguageEnglish
Part of speechNoun
IPA/ˈɛf.ɪ.kə.si/
Letters8
Frequency rank#11,537
Misspellings tracked10
Confusable pairs0
SourceWiktionary (kaikki.org)

Frequency seat of misspelling magnet “efficacy”

This scale runs from the tiny cluster of words that carry ordinary speech (left) to the long tail of words almost nobody reaches for (right).efficacylands here:

#1#100#1K#10K#100K
← used constantlyrarely used →

The axis is logarithmic, with each marked tick roughly ten times rarer than the last. Same rarity band neighbours include “durable” (#11,536), “Erik” (#11,538), “drummer” (#11,535). Among 2,246 “E” headwords, “efficacy” is #11,537. Source: FrequencyWords open word-frequency list.

A misspelling magnet

The generator records 10 spelling variants around efficacy (IPA /ˈɛf.ɪ.kə.si/), anoun. Corpus frequency is #11,537 among 2,246 “E” headwords. Wiktionary lists 2 senses, so context still picks the gloss.

Our generated misspelling index lists 10 likely wrong-spelling variants for efficacy, with forms such as "effciacy", "effiaccy", and "efficaccy". Each variant is a distinct typo pattern an edit-distance generator flags, typically a doubled-consonant error, a silent-letter drop, or a vowel substitution. It is not paired with a close-neighbour confusable in our dataset, a sign it's visually distinctive enough not to be mixed up with another word.

Etymologically, the entry records: From Old French efficace, from Late Latin efficācia (“efficacy”), from efficāx (“efficacious”); see efficacious. The correct English form is efficacy, spelled E-F-F-I-C-A-C-Y.

Definition

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

Recorded use

Wiktionary examples

These source excerpts show how efficacy appears in context. They illustrate usage; the definition above remains the entry’s reference meaning.

  1. […]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.
  2. No method of verification known to science is applicable to the efficacy of prayer. […] If, then, the efficacy of prayer eludes the test of science, and if even uncertainty may rest upon the connection between an event asked in prayer, and the prayer that sought it, is there any evidence by which the efficacy of prayer may be tested and known?

Etymology

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

Synonyms

efficacity

This word in other languages

eficacia(Español)efficience(Français)Wirksamkeit(Deutsch)eficácia(Português)

Common misspellings

Also misspelled as:

  • effciacy
  • effiaccy
  • efficaccy
  • efficacyy
  • efficayc
  • efficcacy
  • efficcay
  • eficacy
  • efifcacy
  • feficacy

Misspelling Pattern Breakdown

How far each generated variant is from the correct spelling of efficacy - expressed in single-character edits (insert, delete, or swap one letter). Bigger bars stand out at a glance; a one-edit slip is the hardest to catch.

effciacy2effiaccy2efficaccy1efficacyy1efficayc2efficcacy1efficcay2eficacy1
Edit distance from "efficacy"

Definitions, pronunciation, and etymology for this entry are drawn from Wiktionary via the kaikki.org structured extract (CC BY-SA); frequency ordering uses the FrequencyWords open word-frequency list (2018 English corpus, MIT). See the methodology for how each field is sourced and updated.

Frequently Asked Questions

How do you spell "efficacy"?
"efficacy" is spelled E-F-F-I-C-A-C-Y. The IPA pronunciation is /ˈɛf.ɪ.kə.si/.
What does "efficacy" mean?
As a noun, "efficacy" means: The ability to produce a desired effect under ideal testing conditions.
What are common misspellings of "efficacy"?
Common misspellings include "effciacy", "effiaccy", "efficaccy", "efficacyy", "efficayc". The correct spelling is "efficacy".
How do you pronounce "efficacy"?
The IPA (International Phonetic Alphabet) transcription for "efficacy" is /ˈɛf.ɪ.kə.si/. Click the speaker icon on the pronunciation badge above to hear it spoken aloud where audio is available.
What is the origin of the word "efficacy"?
From Old French efficace, from Late Latin efficācia (“efficacy”), from efficāx (“efficacious”); see efficacious. See the full etymology section above for more details.
Is PlainSpell free to use?
Yes, PlainSpell is a completely free word reference. You can look up definitions, pronunciations, confusable pairs, homophones, and spelling corrections across 5 languages without any sign-up or subscription.

Similar English words by spelling shape

Same letter count and similar misspelling depth for "efficacy", not the corpus desk frequency band.

Same letter count

Frequency-ranked English headwords with 8 letters (nearest by frequency rank).

debating#11,529Filipino#11,546flashing#11,547granting#11,552

Similar misspelling depth

Headwords with a similar count of attested misspellings (10 here; floor ≥5).

Hawaiian10 variantscommunal10 variantsJupiter10 variantsmutually10 variants

Source: Wiktionary (via kaikki.org) Structured Wiktionary extract

Source: FrequencyWords open word-frequency list