1. quick to notice and respond to danger or opportunity
2. mentally sharp and aware

1. a warning of danger or a call to be ready
2. a state of readiness for danger

1. to warn someone of danger or make them aware of something

Examples

• The guard remained alert throughout the night shift.
• She was mentally alert despite her age.
• The tornado alert warned residents to take shelter.
• The security team was on high alert after the threat.
• The system will alert you when new messages arrive.
• Police alerted the public about the missing child.
• Stay alert while driving in heavy traffic.
• The smoke alarm alert woke everyone in the house.