omen, sign or augury believed to foreshadow the future. Almost any occurrence can be interpreted as an omen. The typical omen was a natural phenomenon, such as a meteor, an eclipse, or the flight of birds. Among the Greeks and Romans the interpretation of omens was a major part of religious life and required trained priests, such as the Roman augur, to explain the meaning of the signs. Belief in omens still survives in superstitions concerning such things as black cats, nightmares, unlucky days, and breaking mirrors.