onlooker vs watcher

onlooker and watcher both are nouns.

Word	Noun	Adjective	Verb	Adverb
onlooker	Yes	No	No	No
watcher	Yes	No	No	No

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

onlooker: someone who looks on
watcher: a close observer; someone who looks at something (such as an exhibition of some kind)

Other hypernyms of onlooker include looker, spectator, viewer, witness.

onlooker (noun)	watcher (noun)
someone who looks on	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