We found 45 dictionaries that define the word efficacy:

General (25 matching dictionaries)

1. efficacy: Merriam-Webster
2. efficacy: Oxford Learner's Dictionaries
3. efficacy: American Heritage Dictionary of the English Language
4. efficacy: Collins English Dictionary
5. efficacy: Vocabulary.com
6. Efficacy, efficacy: Wordnik
7. efficacy: Cambridge Advanced Learner's Dictionary
8. efficacy: Wiktionary
9. efficacy: Webster's New World College Dictionary, 4th Ed.
10. efficacy: The Wordsmyth English Dictionary-Thesaurus
11. efficacy: Infoplease Dictionary
12. Efficacy, efficacy: Dictionary.com
13. efficacy: Online Etymology Dictionary
14. Efficacy (pharmacology), Efficacy: Wikipedia, the Free Encyclopedia
15. Efficacy: Online Plain Text English Dictionary
16. efficacy: Webster's Revised Unabridged, 1913 Edition
17. efficacy: Rhymezone
18. Efficacy: AllWords.com Multi-Lingual Dictionary
19. efficacy: Webster's 1828 Dictionary
20. efficacy: FreeDictionary.org
21. efficacy: Mnemonic Dictionary
22. efficacy: TheFreeDictionary.com
23. efficacy: Longman Dictionary of Contemporary English

Art (1 matching dictionary)

24. Efficacy: Health & Beauty Glossary

Business (2 matching dictionaries)

25. efficacy: Legal dictionary
26. BusinessDictionary.com (No longer online)

Medicine (13 matching dictionaries)

27. Dorland's Illustrated Medical Dictionary (No longer online)
28. AIDSinfo Glossary (No longer online)
29. online medical dictionary (No longer online)
30. Efficacy: Glossary of Terms Used in Medicinal Chemistry
31. Lay Terms for Consent Forms (No longer online)
32. Hair Facts (No longer online)
33. Hepatitis C Information Central (No longer online)
34. GI Disorders (No longer online)
35. Efficacy: Glossary of Medical Education Terms
36. AIDS Medical Glossary and Drug Chart (No longer online)
37. efficacy: Dictionary of Cancer Terms
38. Glossary of HIV/AIDS Related Terms (No longer online)
39. efficacy: Medical dictionary

Science (1 matching dictionary)

40. Efficacy: Medicinal Chemistry

Slang (1 matching dictionary)

41. efficacy: Urban Dictionary

Tech (2 matching dictionaries)

42. Efficacy: Beauty & Health Glossary
43. Glossary of Energy Terms (No longer online)

(Note: See efficacies as well.)

Definitions from Wiktionary ( ) ▸ noun:  The ability to produce a desired effect under ideal testing conditions. ▸ noun:  A degree of ability to produce a desired effect; effectiveness. Similar: efficaciousness, effectivity, efficacity, inefficacy, usefulness, validity, tolerability, potency, effectuality, reliability, more... Opposite: ineffectiveness, inefficacy, uselessness, futility, impotence Types: therapeutic, curative, palliative, prophylactic, more... Phrases: self efficacy, Luminous efficacy, lighting efficacy, Efficacy of prayer, Vaccine efficacy, more... Adjectives: self, clinical, therapeutic, political, relative, causal, greater, term, comparative, personal, collective Colors:     blue,     green,     silver,     gold,     white, more... ▸ Word origin ▸ Words similar to efficacy ▸ Usage examples for efficacy ▸ Idioms related to efficacy ▸ Wikipedia articles (New!) ▸ Popular adjectives describing efficacy ▸ Words that often appear near efficacy ▸ Rhymes of efficacy ▸ Invented words related to efficacy