1. (in military or civilian defense) the most urgent form of alert, signaling that an enemy attack is believed to be imminent.
2. the signal or alarm sounded for this alert. Cf. blue alert, white alert, yellow alert.
3. a signal or warning that a critical situation is developing or has occurred.
4. the period during which a state of crisis or danger is declared to exist.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.