efficacy
/ˈɛf.ɪ.kə.si/
"efficacy" is a 8-letter English headword indexed on PlainSpell.
The verdict
“efficacy” is a moderately-common English word, ranked #11,537 in English word frequency and used as a noun.
- #11,537
- frequency rank, English
- 8
- letters
- 10
- tracked misspellings
According to Wiktionary data (CC BY-SA, analyzed May 6, 2026) - The ability to produce a desired effect under ideal testing conditions.
| Property | Value |
|---|---|
| Headword | efficacy |
| Language | English |
| Part of speech | Noun |
| IPA | /ˈɛf.ɪ.kə.si/ |
| Letters | 8 |
| Frequency rank | #11,537 |
| Misspellings tracked | 10 |
| Confusable pairs | 0 |
| Source | Wiktionary (kaikki.org) |
Where “efficacy” sits in English frequency
Spelling & Dictionary Insight
The English entry for efficacy is 8 letters long, classified as a noun, and transcribed in the International Phonetic Alphabet as /ˈɛf.ɪ.kə.si/. Corpus data places it at rank #11,537 in overall English word frequency, marking it as uncommon enough that many writers pause before typing it. Wiktionary records 2 distinct senses for this headword, so context determines which meaning a reader should apply.
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
- 1The ability to produce a desired effect under ideal testing conditions.
- 2A degree of ability to produce a desired effect; effectiveness.
Etymology
From Old French efficace, from Late Latin efficācia (“efficacy”), from efficāx (“efficacious”); see efficacious.
Synonyms
This word in other languages
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.
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
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Using “efficacy”
The practical upshot for anyone who landed here from a spell-check.
- The one correct English spelling is E-F-F-I-C-A-C-Y - every other letter order is a misspelling in standard orthography.
- Say it as /ˈɛf.ɪ.kə.si/ (IPA); tap the speaker on the pronunciation badge to hear it where audio exists.
- Browse more English words and confusable pairs in the same reference. English words
Data Source
Wiktionary (via kaikki.org), licensed under CC BY-SA & GFDL. Word ordering uses an open word-frequency list; misspelling variants are generated by edit-distance from the correct headword.