Alert /(ȧ*lẽrt")/

A·lert

Alert

a.
  1. Watchful; vigilant; active in vigilance.
  2. Brisk; nimble; moving with celerity.
    An alert young fellow.

Alert

n.
  1. An alarm from a real or threatened attack; a sudden attack; also, a bugle sound to give warning. (Mil.)

Phrases & Compounds
On the alert
on the lookout or watch against attack or danger; ready to act.