To be very attentive and ready to take action. This term is often applied to soldiers. We're safe here—the soldiers patrolling the border are on full alert.As soon as I heard my window break in the middle of the night, I was on full alert to fight off an intruder.
if soldiers are on full alert, they know that a situation is dangerous and are prepared to act immediately if necessary The British flagship in the area went to battle stations and remained on full alert for twenty minutes.