Definition of sentries:
(n) :
A guard, particularly on duty at the entrance to a military base.
(n) :
(uncountable) Sentry duty; time spent being a sentry.
(n) :
An animal like a marmot tasked with alerting the pack to danger.
(n) :
A watchtower.
(n) :
(nautical) A form of drag to be towed underwater, which on striking bottom is upset and rises to the surface.