security guard vs watcher

security guard and watcher both are nouns.

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

As nouns, security guard and watcher are synonyms defined as:

Other synonyms of security guard include watchman.

security guard (noun)	watcher (noun)
a guard who keeps watch	a guard who keeps watch
	a close observer; someone who looks at something (such as an exhibition of some kind)
	a person who keeps a devotional vigil by a sick bed or by a dead body