11 letters · 21 points in Scrabble
adjective marked by qualities giving the power to produce an intended effect; -Aldous Huxley
adjective producing or capable of producing an intended result or having a striking effect; -LewisMumford
originFrom Old French efficacieux, from Latin efficāx (“efficacious”) + -ous, from efficere (“to effect, to accomplish”); see effect.
Etymology from Wiktionary, CC BY-SA 3.0