Source : Webster's Revised Unabridged Dictionary (1913)

Alert \A*lert"\, n. (Mil.)
   An alarm from a real or threatened attack; a sudden attack;
   also, a bugle sound to give warning. ``We have had an
   alert.'' --Farrow.

   {On the alert}, on the lookout or watch against attack or
      danger; ready to act.

Source : WordNet®

on the alert
     adj : watchful and prepared for action; "bird watchers on the
           alert for a rare species"; "cops were on the job and
           caught them red-handed" [syn: {on the alert(p)}, {on
           the job(p)}]