Effectual means producing the result that was intended. An effectual remedy, effectual action, prove effectual.
It is formal and slightly legal, and effective has taken over nearly all its work. The distinction once drawn was that effective describes something that does work and effectual something that does the particular job intended. Note ineffectual, which is far commoner than the positive.
1adjectiveeffectualProducing the result that was intended.
“The new medicine proved effectual, and the fever was gone within two days.”
effectual is a quality of effect.
Latin effectualis comes from effectus, an accomplishment, from efficere, to bring about.
Latin
effectualis
Old French
effectuel
Middle English
effectual
English, today
effectual
From Middle English effectual, effectuel, from Old French effectuel, from Latin effectualis.
Etymology from Wiktionary, CC BY-SA