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]

Moby Thesaurus II by Grady Ward, 1.0:

179 Moby Thesaurus words for "alert":
   Klaxon, Mayday, SOS, active, admonish, advertent, advise, agile,
   agog, air-raid alarm, alarm, alarm bell, alarm clock, alarm signal,
   alarum, alert for, alive, all clear, all ears, all eyes, animated,
   apt, arouse, assiduous, attentive, awake, aware, beacon,
   blinking light, brainy, breathe, bright, brilliant, burglar alarm,
   buzz, buzzer, cant, careful, caution, cautious, clear-sighted,
   clear-witted, clearheaded, clever, concentrated, confide,
   confide to, conscious, crostarie, cry havoc, cry wolf, decisive,
   diligent, earnest, entrust with information, equal to, expeditious,
   fiery cross, finical, finicking, finicky, fire alarm, fire bell,
   fire flag, five-minute gun, flashing light, fly storm warnings,
   fog bell, fog signal, foghorn, forewarn, frighten, frisky,
   full of life, gale warning, gay, give confidential information,
   give fair warning, give notice, give warning, heedful, hooter,
   horn, hue and cry, hurricane warning, immediate, instant,
   instantaneous, intense, intent, intentive, issue an ultimatum,
   keen, knowing, let in on, let next to, lighthouse, lively,
   loaded for, lookout, mention privately, mercurial, meticulous,
   mindful, nice, niggling, nimble, note of alarm, notify, observant,
   observing, occulting light, on guard, on the, on the alert,
   on the ball, on the job, on the lookout, open-eared, open-eyed,
   openmouthed, police whistle, prepared for, prompt, punctual,
   put hep, put next to, qui vive, quick, quick-witted, ready,
   ready for, regardful, sharp, signal, signal of distress, siren,
   sleepless, small-craft warning, smart, sound the alarm,
   sound the tocsin, speedy, spirited, sprightly, spry, startle,
   still alarm, storm cone, storm flag, storm warning, summary, swift,
   threaten, tip, tip off, tocsin, two-minute gun, unblinking,
   unnodding, unsleeping, unwinking, up, up for, up to,
   upside-down flag, utter a caveat, vigilant, vivacious, wakeful,
   warn, warn against, warning, wary, watchful, whisper, whistle,
   wide awake, wide-awake



The Free On-line Dictionary of Computing (30 December 2018):

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)