fire watcher vs security guard

fire watcher and security guard both are nouns.

Word	Noun	Adjective	Verb	Adverb
fire watcher	Yes	No	No	No
security guard	Yes	No	No	No

As nouns, security guard is a hypernym of fire watcher; that is, security guard is a word with a broader meaning than fire watcher:

fire watcher: (during World War II in Britain) someone whose duty was to watch for fires caused by bombs dropped from the air
security guard: a guard who keeps watch

Other hypernyms of fire watcher include watcher, watchman.

fire watcher (noun)	security guard (noun)
(during World War II in Britain) someone whose duty was to watch for fires caused by bombs dropped from the air	a guard who keeps watch