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.

Alert \A*lert"\ ([.a]*l[~e]rt"), a. [F. alerte, earlier [`a]
   l'erte on the watch, fr. It. all' erta on the watch, prop.
   (standing) on a height, where one can look around; erta a
   declivity, steep, erto steep, p. p. of ergere, erigere, to
   erect, raise, L. erigere. See {Erect}.]
   1. Watchful; vigilant; active in vigilance.

   2. Brisk; nimble; moving with celerity.

            An alert young fellow.                --Addison.

   Syn: Active; agile; lively; quick; prompt.

Source : WordNet®

alert
     adj 1: very attentive or observant; "an alert and responsive baby
            is a joy"; "caught by a couple of alert cops"; "alert
            enough to spot the opportunity when it came";
            "constantly alert and vigilant, like a sentinel on
            duty" [ant: {unalert}]
     2: mentally responsive; "an alert mind"
     3: not unconscious; especially having become conscious; "the
        patient is now awake and alert" [syn: {awake(p)}]
     4: (usually followed by `to') showing acute awareness; mentally
        perceptive; "alert to the problems"; "alive to what is
        going on"; "awake to the dangers of her situation"; "was
        now awake to the reality of his predicament" [syn: {alive(p)},
         {awake(p)}]

alert
     n 1: condition of heightened watchfulness or preparation for
          action; "bombers were put on alert during the crisis"
          [syn: {qui vive}]
     2: a warning serves to make you more alert to danger [syn: {alerting}]
     3: an automatic signal (usually a sound) warning of danger
        [syn: {alarm}, {warning signal}, {alarum}]

alert
     v : warn or arouse to a sense of danger or call to a state of
         preparedness; "The empty house alarmed him"; "We alerted
         the new neighbors to the high rate of burglaries" [syn: {alarm}]

Source : Free On-Line Dictionary of Computing

alert
     
         /*'l*rt/ An audible and/or visual message
        intended to inform a system's users or administrators about a
        change in the operating conditions of that system or about
        some kind of error condition.  In a {graphical user
        interface}, an alert would typically be displayed as a small
        window containing the message and a button to click to dismiss
        the window.
     
        (1999-03-29)