The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. Brisk; nimble; moving with celerity.
      [1913 Webster]

            An alert young fellow.                --Addison.
      [1913 Webster]

   Syn: Active; agile; lively; quick; prompt.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

   On the alert, on the lookout or watch against attack or
      danger; ready to act.
      [1913 Webster]

WordNet (r) 3.0 (2006):

alert
    adj 1: engaged in or accustomed to close observation; "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" [syn: alert,
           watchful] [ant: unalert, unvigilant, unwatchful]
    2: quick and energetic; "a brisk walk in the park"; "a lively
       gait"; "a merry chase"; "traveling at a rattling rate"; "a
       snappy pace"; "a spanking breeze" [syn: alert, brisk,
       lively, merry, rattling, snappy, spanking, zippy]
    3: mentally perceptive and responsive;"an alert mind"; "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: alert, alive(p), awake(p)]
    n 1: condition of heightened watchfulness or preparation for
         action; "bombers were put on alert during the crisis" [syn:
         alert, qui vive]
    2: a warning serves to make you more alert to danger [syn:
       alert, alerting]
    3: an automatic signal (usually a sound) warning of danger [syn:
       alarm, alert, warning signal, alarum]
    v 1: 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, alert]

The Free On-line Dictionary of Computing (19 January 2023):

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)