viewer vs watcher

viewer and watcher both are nouns.

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

As nouns, viewer and watcher are synonyms defined as:

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

Other synonyms of viewer include looker, spectator, witness.

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