From Longman Dictionary of Contemporary Englishsentrysen‧try /ˈsentri/ noun (plural sentries) [countable] PMa soldier standing outside a building as a guard
Examples from the Corpus
sentry• A shrill whistle is blown angrily by a shivering soldier, a sentry at the tomb.• Soldiers set up barbed-wire fences, electricians wired up searchlights, carpenters built barracks and sentry boxes on elevated platforms.• On the top was a pole wrapped in straw which could be ignited by the cossack sentry in case of enemy incursion.• Or stroll up to one of the distinctive sentry boxes, aligned so defenders could harass would-be invaders with a withering cross-fire.• The new sentry on the northern side was a twenty-one-year-old Berliner called Manfred.• Take sentries - they sit squinting into the dark, waiting to see something move, and naturally their eyes droop.• The sentry sat hugging his rifle.Origin sentry (1600-1700) Perhaps from sentry “sanctuary, guard-tower” ((1600-1700)), from sanctuary 