One who watches or guards; specifically (Mil.), a soldier set to guard an army, camp, or other place, from surprise, to observe the approach of danger, and give notice of it; a sentry.

Watch; guard.

To watch over like a sentinel.

To furnish with a sentinel; to place under the guard of a sentinel or sentinels.

Soldier who keeps guard

One or more members of a group of birds that are perched high above the rest of a group and seem to be watching for trouble. For example, crows are often seen sitting at the top of a tree and watching.

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

a person employed to watch for something to happen

a guard who stands watch and protects a specific place