Any phenomenon or circumstance purporting to portend good or evil. In order to believe in omens a specific prior belief is absolutely necessary that the future is knowable, in other words, that everything that is to be has been foreordained. Thus an omen is an event that presupposes destiny. The chief feature of an omen is its fortuitous and unsought nature: the black cat runs across your path, the owl hoots as you pass by. From far back in recorded history many changing aspects of nature have been noted as harbingers of good or ill; it is interesting that most forebode ill. The probable origin of superstitions of this kind was mankind’s attempt to know the future and perhaps, through fore-knowledge, to avoid disaster. Omens have always formed a basis for action rather than an indication of inexorable fate.
In the ancient world an omen did not have to be an extraordinary happening, like a comet foretelling misfortune. Quite ordinary events observed, or heard, to happen in the sky were noted: thunder and lightning, or the flight or song of birds, could be deemed to be auguries. Very important was the direction from which a sign first came; it was this that indicated whether it predicted good or evil. In Ancient Greece an owl hooting on the left was an unlucky omen while one hooting on the right was lucky; in Roman society the values were reversed. The anomaly could have arisen from Rome’s early national struggles for power in the Mediterranean, leading to the belief that an omen that was bad for Greece was necessarily good for Rome.