alarum vs burglar alarm

alarum and burglar alarm both are nouns.

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

As nouns, burglar alarm is a hyponym of alarum; that is, burglar alarm is a word with a more specific, narrower meaning than alarum:

Other hyponyms of alarum include torpedo, air alert, fire alarm, foghorn, fogsignal, horn, red flag, siren, alarm bell, tocsin.

alarum (noun)	burglar alarm (noun)
an automatic signal (usually a sound) warning of danger	a warning device that is tripped off by the occurrence of a burglary
	a loud warning signal produced by a burglar alarm