watcher vs sentry

watcher and sentry both are nouns.

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

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

watcher (noun)	sentry (noun)
a guard who keeps watch	a person employed to keep watch for some anticipated event
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