From The Collaborative International Dictionary of English v.0.48 [gcide]:
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]
From WordNet (r) 3.0 (2006) [wn]:
alarum
n 1: an automatic signal (usually a sound) warning of danger
[syn: alarm, alert, warning signal, alarum]