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An omen (also called portent or presage) is a phenomenon that is believed to foretell the future, often signifying the advent of change. Though the word "omen" is usually devoid of reference to the change's nature, hence being possibly either "good" or "bad," the term is more often used in a foreboding sense, as with the word "ominous". The origin of the word is unknown, although it may be connected with the Latin word audire, meaning "to hear."
Ancient Roman religion employed two distinct types of professional omen readers. Augurs interpreted the flights of birds, while haruspices employed animal sacrifice to obtain the entrails necessary for divination.
In the field of astrology, solar and lunar eclipses (along with the appearance of comets and to some extent the full moon) have often been considered omens of notable births, deaths, or other significant events throughout history in many societies. One biblical example is the Magi in the Gospel of Matthew who predicted the birth of Jesus after seeing the Star of Bethlehem.
Good or bad
Omens may be considered either good or bad depending on their interpretation. The same sign may be interpreted differently by different people or different cultures.
Comets also have been considered to be both good and bad omens. The best-known example is probably Halley's Comet, which was a "bad omen" for King Harold II of England but a "good omen" for William the Conqueror.