observer vs watcher

observer and watcher both are nouns.

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

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

observer: a person who becomes aware (of things or events) through the senses
watcher: a close observer; someone who looks at something (such as an exhibition of some kind)

observer (noun)	watcher (noun)
a person who becomes aware (of things or events) through the senses	a guard who keeps watch
an expert who observes and comments on something	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