A momentous event, change, or decision is very important because it will shape what comes next. Leaving home, a peace deal, a vote that turns a country — those can be momentous. The word is formal and serious; speeches and history writing love it. It is not about how long something lasts. Though actually, the moment inside this word is an older meaning: weight, importance, not a tick of the clock. So a momentous silence is a silence that matters a great deal, not a short one. Students often reach for this word when they only mean “a moment,” and it comes out too heavy. Save it for things whose later effect is large.