1. [n]  a device that signals the occurrence of some undesirable event
2. [n]  a clock that wakes sleeper at preset time
3. [n]  an automatic signal (usually a sound) warning of danger
4. [n]  fear resulting from the awareness of danger
5. [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"
6. [v]  fill with apprehension or alarm; cause to be unpleasantly surprised; "I was horrified at the thought of being late for my interview"; "The news of the executions horrified us"

ALARM is a 5 letter word that starts with A.

1. \A*larm"\ ([.a]*l[aum]rm"), n. [F. alarme, It. all' arme
   to arms ! fr. L. arma, pl., arms. See {Arms}, and cf.
   {Alarum}.]
   1. A summons to arms, as on the approach of an enemy.
            Arming to answer in a night alarm.    --Shak.
   2. Any sound or information intended to give notice of
      approaching danger; a warning sound to arouse attention; a
      warning of danger.
            Sound an alarm in my holy mountain.   --Joel ii. 1.
   3. A sudden attack; disturbance; broil. [R.] ``These home
      alarms.'' --Shak.
            Thy palace fill with insults and alarms. --Pope.
   4. Sudden surprise with fear or terror excited by
      apprehension of danger; in the military use, commonly,
      sudden apprehension of being attacked by surprise.
            Alarm and resentment spread throughout the camp.
                                                  --Macaulay.
   5. A mechanical contrivance for awaking persons from sleep,
      or rousing their attention; an alarum.
   {Alarm bell}, a bell that gives notice on danger.
   {Alarm clock} or {watch}, a clock or watch which can be so
      set as to ring or strike loudly at a prearranged hour, to
      wake from sleep, or excite attention.
   {Alarm gauge}, a contrivance attached to a steam boiler for
      showing when the pressure of steam is too high, or the
      water in the boiler too low.
   {Alarm post}, a place to which troops are to repair in case
      of an alarm.
   Syn: Fright; affright; terror; trepidation; apprehension;
        consternation; dismay; agitation; disquiet; disquietude.
   Usage: {Alarm}, {Fright}, {Terror}, {Consternation}. These
          words express different degrees of fear at the
          approach of danger. Fright is fear suddenly excited,
          producing confusion of the senses, and hence it is
          unreflecting. Alarm is the hurried agitation of
          feeling which springs from a sense of immediate and
          extreme exposure. Terror is agitating and excessive
          fear, which usually benumbs the faculties.
          Consternation is overwhelming fear, and carries a
          notion of powerlessness and amazement. Alarm agitates
          the feelings; terror disorders the understanding and
          affects the will; fright seizes on and confuses the
          sense; consternation takes possession of the soul, and
          subdues its faculties. See {Apprehension}.
   

2. \A*larm"\, v. t. [imp. & p. p. {Alarmed}; p. pr. & vb. n.
   {Alarming}.] [Alarm, n. Cf. F. alarmer.]
   1. To call to arms for defense; to give notice to (any one)
      of approaching danger; to rouse to vigilance and action;
      to put on the alert.
   2. To keep in excitement; to disturb.
   3. To surprise with apprehension of danger; to fill with
      anxiety in regard to threatening evil; to excite with
      sudden fear.
            Alarmed by rumors of military preparation.
                                                  --Macaulay.
   

a particular quivering sound of the silver trumpets to give warning to the Hebrews on their journey through the wilderness (Num. 10:5, 6), a call to arms, or a war-note (Jer. 4:19; 49:2; Zeph. 1:16).