Longman Dictionary of Contemporary English homepage

Date: 1500-1600
Language: French
Origin: alerte, from Italian all' erta 'on the watch'

alert

1 adjective
     
a‧lert1
1 giving all your attention to what is happening, being said etc:
The animal raised its head, suddenly alert.
Taking notes is one of the best ways to stay alert in lectures.
2 able to think quickly and clearly:
Jack was as mentally alert as a man half his age.
3

be alert to something

to know about or understand something, especially a possible danger or problem:
The authorities should have been alert to the possibility of invasion.
alertness noun [uncountable]

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