The Collaborative International Dictionary of English v.0.48:

Alarum \A*lar"um\ (?; 277), n. [OE. alarom, the same word as
   alarm, n.]
   See Alarm. [Now Poetic]
   [1913 Webster]

   Note: The variant form alarum is now commonly restricted to
         an alarm signal or the mechanism to sound an alarm (as
         in an alarm clock.)
         [1913 Webster]

WordNet (r) 3.0 (2006):

alarum
    n 1: an automatic signal (usually a sound) warning of danger
         [syn: alarm, alert, warning signal, alarum]