1. [n]  an automatic signal (usually a sound) warning of danger
2. [n]  a warning serves to make you more alert to danger
3. [n]  condition of heightened watchfulness or preparation for action; "bombers were put on alert during the crisis"
4. [adj]  not unconscious; especially having become conscious; "the patient is now awake and alert"
5. [adj]  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"
6. [adj]  (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"
7. [adj]  mentally responsive; "an alert mind"
8. [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"

ALERT is a 5 letter word that starts with A.

1. \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.
   

2. \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.
   

/*'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.