onlooker vs viewer

onlooker and viewer both are nouns.

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

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

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

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

onlooker (noun)	viewer (noun)
someone who looks on	an optical device for viewing photographic transparencies
	a close observer; someone who looks at something (such as an exhibition of some kind)