a·lert / əˈlərt/ • adj. quick to notice any unusual and potentially dangerous or difficult circumstances; vigilant. ∎ able to think clearly; intellectually active.• n. the state of being watchful for danger: forces placed on alert. ∎ an announcement or signal warning of danger: a bomb alert. ∎ a period of vigilance in response to such a warning: traffic was halted during the alert.• v. [tr.] warn (someone) of a danger, threat, or problem, typically with the intention of having it avoided or dealt with: he alerted people to the dangers of smoking police were alerted after three men drove away without paying.PHRASES: on the alert vigilant and prepared.DERIVATIVES: a·lert·ly adv.a·lert·ness n.