- 1able to think quickly; quick to notice things Suddenly he found himself awake and fully alert. Two alert scientists spotted the mistake. Try to stay alert while driving at night. an alert mind mentally alert
- 2alert to something aware of something, especially a problem or danger We must be alert to the possibility of danger. She was alert to the slightest sound.
alertly adjectivejump to other results
alertness noun [uncountable]jump to other results