1. (in military or civilian defense) the first alert given when enemy aircraft are discovered approaching a military installation, city, coastline, etc. Cf. blue alert, red alert, white alert.
2. the signal or alarm sounded for this alert.
3. any primary stage of alert, as when toxic chemicals have escaped in a particular area and a potential hazard exists.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.