Refers to a soldier or guard assigned to keep watch, often while walking a patrol route. Common in military contexts.
A sentry was patrolling around the base.
At night, the sentry's vigilance becomes stricter.
Compound of 歩 (walk) and 哨 (watch, guard), literally 'walking guard'. The exact historical derivation is uncertain, but the kanji combination directly reflects the meaning of a sentry on patrol.