watcher vs onlooker

watcher and onlooker both are nouns.

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

As nouns, onlooker is a hyponym of watcher; that is, onlooker is a word with a more specific, narrower meaning than watcher:

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

watcher (noun)	onlooker (noun)
a guard who keeps watch	someone who looks on
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