Efficacy

How to spell efficacy

8 letters · 4 syllables

How to pronounce efficacy

Say the part in capitals loudest.

What does efficacy mean?

1. nounThe power or ability of something to produce the intended result.

Efficacy in a sentence

• Clinical trials measured the efficacy of the new vaccine.

• The report questioned the efficacy of the marketing campaign.

Where does efficacy come from?

Language of origin: Latin

This word comes from the Latin efficacia, meaning 'power to produce an effect', which was built on the adjective efficax 'effective, capable of accomplishing'. Both go back to the verb efficere, 'to bring about' or 'to accomplish', formed from ex- 'out, thoroughly' and facere 'to do or make'. English took the word up in the early 1500s, at first in medical and theological writing about whether a remedy or a grace genuinely worked. The same facere root sits at the heart of a huge family of English words, including effect, efficient, factory, and manufacture, all of which share the idea of making something happen. Knowing that the middle is the 'fic' from facere helps explain why the spelling keeps the single c and an a before the -cy, giving the slightly tricky -cacy ending.

How the word is built

• ef- (from ex-, out/thoroughly)
• fic (from facere: to do, make)
• -acy (noun-forming suffix denoting quality or state)