A sentry is a guard who stands watch at a specific post, keeping alert for danger or unwelcome visitors. Think of a soldier stationed at the entrance to a military base, scanning the horizon for threats, or a guard posted outside a castle gate in medieval times. The sentry's job is to stay vigilant and raise an alarm if something's wrong.

In historical fortresses, sentries would pace along the walls through the night, making sure no enemies approached under cover of darkness. Today, you might find sentries guarding important government buildings, military installations, or high-security facilities.

A sentry differs from a general guard because they have a specific, focused assignment: stand here, watch there, and alert others if needed. When someone is described as standing sentry, they're keeping careful watch over something, even if they're not literally a guard. A student who volunteers to watch for the teacher's return while classmates finish a project is standing sentry in a small way.