The Collaborative International Dictionary of English v.0.48:

Alarm \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.
      [1913 Webster]

            Arming to answer in a night alarm.    --Shak.
      [1913 Webster]

   2. Any sound or information intended to give notice of
      approaching danger; a warning sound to arouse attention; a
      warning of danger.
      [1913 Webster]

            Sound an alarm in my holy mountain.   --Joel ii. 1.
      [1913 Webster]

   3. A sudden attack; disturbance; broil. [R.] "These home
      alarms." --Shak.
      [1913 Webster]

            Thy palace fill with insults and alarms. --Pope.
      [1913 Webster]

   4. Sudden surprise with fear or terror excited by
      apprehension of danger; in the military use, commonly,
      sudden apprehension of being attacked by surprise.
      [1913 Webster]

            Alarm and resentment spread throughout the camp.
                                                  --Macaulay.
      [1913 Webster]

   5. A mechanical contrivance for awaking persons from sleep,
      or rousing their attention; an alarum.
      [1913 Webster]

   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.
      [1913 Webster]

   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.
          [1913 Webster]

WordNet (r) 3.0 (2006):

alarm bell
    n 1: the sound of an alarm (usually a bell) [syn: tocsin,
         alarm bell]