Longman Dictionary of Contemporary English homepage

alert

3 noun
     
alert3
1 [countable] a warning to be ready for possible danger
a bomb/fire/terrorist etc alert
a full-scale flood alert
The bomb alert was raised soon after midnight.
 ➔ red alert
2

on (the) alert (for something/somebody)

ready to notice and deal with a situation or problem:
Be on the alert for anyone acting suspiciously.
Troops in the vicinity were put on alert.
on full alert also on high alert (=completely ready to deal with a dangerous situation)
All our border points are on full alert.
Definition from the Longman Dictionary of Contemporary English
Advanced Learner's Dictionary.

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