onlooker vs spectator

onlooker and spectator both are nouns.

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

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

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

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

onlooker (noun)	spectator (noun)
someone who looks on	a woman's pump with medium heel; usually in contrasting colors for toe and heel
	a close observer; someone who looks at something (such as an exhibition of some kind)