What does efficacity mean?
Efficacity means the ability to produce a desired effect — in plain terms, how well something actually works. It is essentially a synonym of 'efficacy' and 'effectiveness', but unlike them it has fallen out of common use, surviving mainly in older texts or in French-influenced writing. A medicine's efficacity, for instance, would be its proven power to cure or relieve a condition, judged on whether it achieves the intended outcome rather than on how economically it does so. Because the word looks unfamiliar to modern readers, using it risks confusion with 'efficiency', which concerns resourcefulness rather than raw effectiveness. For everyday and formal writing alike, 'efficacy' or 'effectiveness' are safer choices; 'efficacity' is best reserved for historical quotation or deliberate stylistic flourish. Its rarity makes it a curiosity of English vocabulary: a perfectly serviceable word whose shorter rival simply won out over centuries of usage.
nounThe quality or degree of being effective; the power to produce a desired result.
- The capacity to produce a desired effect or result; effectiveness.
"The committee praised the vaccine's remarkable efficacity against the new variant."
"Doctors were struck by the treatment's efficacity even in the most severe cases."
"In his report, the auditor questioned the efficacity of the new training programme."
Almost never used; the plural appears only when comparing distinct kinds or instances of effectiveness, typically in older or academic prose.
"The report compared the efficacities of several competing methods of water purification."
Efficacity once fought 'efficacy' for a place in the dictionary — and lost so decisively that most spellcheckers flag it as an error today.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of efficacity
Efficacity derives from the Latin adjective efficax (genitive efficacis), meaning 'effective', from efficere, 'to bring about, accomplish', formed from ex- ('out') plus facere ('to make or do'). It entered English in the late Middle Ages alongside its sibling 'efficacy', both modelled on Latin-derived forms. Over time, the shorter 'efficacy' became the standard English choice, while 'efficacity' endured more strongly in French (efficacité) and other Romance languages, leaving the English form rare and somewhat archaic.
How efficacity is actually used
Efficacity is rare in contemporary English and may strike many readers as archaic or erroneous. In formal, scientific, and medical writing, 'efficacy' is overwhelmingly preferred; use 'efficacity' only deliberately, for stylistic or historical effect.
Easily confused with efficacity
'Efficacy' is the standard modern term for the power to produce an effect; 'efficacity' is an older, rarer synonym that now sounds archaic.
'Efficiency' refers to achieving results without wasted effort or resources; 'efficacity' concerns only whether the desired result is produced at all.