fully aware and attentive; wide-awake; keen

an alert mind.

swift; agile; nimble.

an attitude of vigilance, readiness, or caution, as before an expected attack.

a warning or alarm of an impending military attack, a storm, etc.

We'd just boarded the bus when the alert sounded.

the period during which such a warning or alarm is in effect.

to warn (troops, ships, etc.) to prepare for action.

to warn of an impending raid, attack, storm, etc.

The radio alerted coastal residents to prepare for the hurricane.

to advise or warn; cause to be on guard

to alert gardeners to the dangers of some pesticides.

on the alert

on guard against danger; in readiness; vigilant

The state police are on the alert for an escaped convict believed to be in the area.

noun

sharpness , readiness , attentiveness , vigilance , wakefulness , wariness , watchfulness