a person or thing that watches or stands as if watching.

a soldier stationed as a guard to challenge all comers and prevent a surprise attack

to stand sentinel.

Also called tag. Computers . a symbol, mark, or other labeling device indicating the beginning or end of a unit of information.

to watch over or guard as a sentinel.

noun

guard , keeper , lookout , picket , protector , watchman/woman , watchperson , sentry , ward , watch , patrol , perdue.associated word: cordon , soldier , vidette , watchman