alarum vs alarm

alarum and alarm both are nouns.

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

As nouns, alarum and alarm are synonyms defined as:

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

Other synonyms of alarum include alert, warning signal.

alarum (noun)	alarm (noun)
an automatic signal (usually a sound) warning of danger	a device that signals the occurrence of some undesirable event
	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

alarum (verb)	alarm (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