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English definition of “alert”

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alert

adjective uk   /əˈlɜːt/ us    /-ˈlɝːt/
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   /-nəs/ us  

alert

noun [C or U] uk   /əˈlɜːt/ us    /-ˈlɝːt/
C1 a warning to people to be prepared to deal with something dangerous: a bomb alert The army was put on (full) alert as the peace talks began to fail. The public were warned to be on the alert for (= watching carefully for) suspicious packages.

alert

verb [T] uk   /əˈlɜːt/ us    /-ˈlɝːt/
C2 to warn someone of a possibly dangerous situation: An anonymous letter alerted police to the possibility of a terrorist attack at the airport.
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(Definition of alert from the Cambridge Advanced Learners Dictionary & Thesaurus © Cambridge University Press)
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