Dictionary of English - effective, effectual, efficacious, or efficient
effective, effectual, efficacious, or efficient
If something is effective it has an noticeable effect: The actor made a most effective entrance. If it is effectual it produces a particular effect, usually the one intended: We took effectual steps to redress the situation. If a thing is efficacious it has the power or potential to produce a particular effect: These tablets are efficacious against malaria. If a thing is efficient it works well: Josie did a very efficient job with the lawns.