The Oxford Thesaurus An A-Z Dictionary of Synonyms - effectual adj.

1) effective, efficacious, efficient, functional, productive, useful, telling, influential, powerful, forcible, forceful; capable, adequate: What is the most effectual way of stopping a take-over of the company? 2 effective, in force, legal, lawful, binding, sound, valid: You must register the agreement for it to be effectual.