rubberneck vs viewer

rubberneck and viewer both are nouns.

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

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

rubberneck: a person who stares inquisitively
viewer: a close observer; someone who looks at something (such as an exhibition of some kind)

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

rubberneck (noun)	viewer (noun)
a person who stares inquisitively	an optical device for viewing photographic transparencies
a tourist who is visiting sights of interest	a close observer; someone who looks at something (such as an exhibition of some kind)

rubberneck (verb)	viewer (verb)
strain to watch; stare curiously