A sign that something will happen if nothing happens.

An occurrence supposed to portend, or show the character of, some future event; any indication or action regarded as a foreshowing; a foreboding; a presage; an augury.

To divine or to foreshow by signs or portents; to have omens or premonitions regarding; to predict; to augur; as, to omen ill of an enterprise.

Any phenomenon or thing that is interpreted as a sign of good luck or misfortune.