sentry

noun, plural sen·tries.

1. a soldier stationed at a place to stand guard and prevent the passage of unauthorized persons, watch for fires, etc., especially a sentinel stationed at a pass, gate, opening in a defense work, or the like.
2. a member of a guard or watch.

noun plural -tries

1. a soldier who guards or prevents unauthorized access to a place, keeps watch for danger, etc
2. the watch kept by a sentry

n.1610s, originally “watchtower;” perhaps a shortened variant of sentinel, which had a variant form centrinel (1590s); or perhaps worn down from sanctuary, on notion of “shelter for a watchman.” Meaning “military guard posted around a camp” is first attested 1630s. Sentry-box is from 1728.