/ Thesaurus / E Synonyms of efficaciousness Find synonyms for: Noun1. efficacy, efficaciousness, effectiveness, effectivity, effectualness, effectualityusage: capacity or power to produce a desired effect; "concern about the safety and efficacy of the vaccine"WordNet 3.0 Copyright © 2006 by Princeton University. All rights reserved. Related Content Daily Word Quiz: languid Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language