If you refer to a decision, event, or change as momentous, you mean that it is very important, often because of the effects that it will have in the future. the momentous decision to send in the troops. a momentous event, change, or decision is very important because it will have a great influence on the future
Of moment or consequence; very important; weighty; as, a momentous decision; momentous affairs
of very great significance; "deciding to drop the atom bomb was a very big decision"; "a momentous event"