Meaning of “alert” in the English Dictionary

"alert" in British English

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alertadjective

uk /əˈlɜːt/ us /əˈlɝːt/

C1 quick to see, understand, and act in a particular situation:

I'm not feeling very alert today - not enough sleep last night!
A couple of alert readers posted comments on the website pointing out the mistake.
Parents should be alert to sudden changes in children's behaviour.

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alertness
noun [ U ] uk /əˈlɜːt.nəs/ us /əˈlɝːt.nəs/

alertnoun [ C or U ]

uk /əˈlɜːt/ us /əˈlɝːt/

alertverb [ T ]

uk /əˈlɜːt/ us /əˈlɝːt/

(Definition of “alert” from the Cambridge Advanced Learner’s Dictionary & Thesaurus © Cambridge University Press)

"alert" in American English

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alertadjective

us /əˈlɜrt/

quick to see, understand, and act in a particular situation:

When you’re driving, you must stay alert.
We had to be alert to any danger signs in the economy.

alertnoun [ C/U ]

us /əˈlɜrt/

a warning to people to get ready to deal with something dangerous:

[ U ] The police were on the alert for (= watching carefully for) any sign of trouble.
alert
verb [ T ] us /əˈlɜrt/

The auto company mailed letters to owners of that model car, alerting them of safety risks.

(Definition of “alert” from the Cambridge Academic Content Dictionary © Cambridge University Press)