spotter vs sentry

spotter and sentry both are nouns.

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

As nouns, spotter and sentry are synonyms defined as:

spotter and sentry: a person employed to keep watch for some anticipated event

Other synonyms of spotter include lookout, lookout man, picket, scout, sentinel, watch.

spotter (noun)	sentry (noun)
someone who is the first to observe something	a person employed to keep watch for some anticipated event
a worker employed to apply spots (as markers or identifiers)
a worker employed at a dry-cleaning establishment to remove spots
a person employed to keep watch for some anticipated event