sentry

Short definition from Wiktionary

Noun

1. 1
   A guard, particularly on duty at the entrance to a military base.
2. 2
   Sentry duty; time spent being a sentry.
3. 3
   A form of drag to be towed underwater, which on striking bottom is upset and rises to the surface.
4. 4
   A watchtower.
5. 5
   An animal like a marmot tasked with alerting the pack to danger.