Sentry (noun)

1. Noun: a person, such as a soldier or guard, posted to keep watch.
2. Noun: a person or thing that guards or watches over something.

Origin:

From latin word "sentinēla" meaning "watchman" or "guard".

Examples:

1. The sentry at the gate checked our IDs before letting us enter.
2. The sentry stood at attention, keeping watch over the palace.
3. The sentry towers were placed strategically around the perimeter of the castle.
4. The sentry was on high alert, ready to sound the alarm at the first sign of danger.
5. The sentry watched over the city, keeping it safe from invaders.