Watchful; vigilant; active in vigilance.
- Vigilantly attentive; watchful: alert to danger; an alert bank guard. See Synonyms at aware.
- Mentally responsive and perceptive; quick.
- Brisk or lively in action: the bird's alert hopping from branch to branch.
- A signal that warns of attack or danger: Sirens sounded the alert for an air raid.
- A condition or period of heightened watchfulness or preparation for action: Nuclear-armed bombers were put on alert during the crisis.
- To notify of approaching danger or action; warn: a flashing red light that alerted motorists to trouble ahead.
- on the alert Watchful and prepared for danger, emergency, or opportunity: bird watchers on the alert for a rare species.