Source : Webster's Revised Unabridged Dictionary (1913)

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

   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.)

Source : WordNet®

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