adjective

Quick to notice any unusual and potentially dangerous or difficult circumstances; vigilant.

an alert police officer discovered a truck full of explosives

vigilant,wide awake,aware,watchful,attentive,observant,circumspect,wary,chary,heedful,canny

noun

The state of being watchful for possible danger.

security forces were placed on alert

vigilance,watchfulness,carefulness,attentiveness,guardedness,care,caution,cautiousness,wariness,chariness,alertness,circumspection,prudence,heedfulness,heed,mindfulness

transitive verb

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

warn,notify,apprise,caution,put on one’s guard,forewarn,put on the qui vive,arouse