omen

noun
  1. An event or phenomenon believed to foretell the future, often signifying a change or an important event. noun
    The sudden appearance of a comet was considered an omen of great change.
    Finding a four-leaf clover is often seen as a good omen.
  2. A sign or indication of something that is going to happen, especially something momentous or calamitous. noun
    The dark clouds on the horizon were an ominous omen of the approaching storm.
    The ancient ruins were interpreted as an omen of past civilizations.