Synonyms for Effectual (same or very similar meaning)
WordNet sense 1 (having legal efficacy or force):
legal, sound
WordNet sense 2 (well grounded in logic or truth or having legal force):
valid
WordNet sense 3 (producing a strong effect):
telling
WordNet sense 4 (producing or capable of producing an intended result or having a striking effect):
effective, efficacious
From the ODE community, based on WordNet
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