Definition of "sentry"

A guard, particularly on duty at the entrance to a military base.  quotations  examples

(uncountable) Sentry duty; time spent being a sentry.  examples

(nautical) A form of drag to be towed underwater, which on striking bottom is upset and rises to the surface.  examples

A watchtower.  examples

An animal like a marmot tasked with alerting the pack to danger.  examples