alarm vs alarum

alarm and alarum both are nouns.

Word	Noun	Adjective	Verb	Adverb
alarm	Yes	No	Yes	No
alarum	Yes	No	No	No

As nouns, alarm and alarum are synonyms defined as:

alarm and alarum: an automatic signal (usually a sound) warning of danger

Other synonyms of alarm include alert, warning signal.

alarm (noun)	alarum (noun)
a device that signals the occurrence of some undesirable event	an automatic signal (usually a sound) warning of danger
a clock that wakes a sleeper at some preset time
an automatic signal (usually a sound) warning of danger
fear resulting from the awareness of danger

alarm (verb)	alarum (verb)
warn or arouse to a sense of danger or call to a state of preparedness
fill with apprehension or alarm; cause to be unpleasantly surprised