Momentous/məʊˈmentəs/ Definition 1 of 1Pro Tip 1/3Lasting Impact Use 'momentous' for events with long-lasting influence on people or society. The Supreme Court's ruling was a momentous achievement for civil rights. Pro Tip 2/3Not Everyday Use 'Momentous' is reserved for rare, exceptional happenings, not common occurrences. Graduating from university is a momentous occasion for many. Pro Tip 3/3Positive or Negative Can describe both positive and negative events, if they are very impactful. The earthquake was a momentous disaster, changing the landscape forever.