Sentinel

1. Watch; guard.
2. To furnish with a sentinel; to place under the guard of a sentinel or sentinels.
3. To watch over like a sentinel.
4. A marine crab (Podophthalmus vigil) native of the Indian Ocean, remarkable for the great length of its eyestalks; -- called also sentinel crab.
5. One who watches or guards; specifically (Mil.), a soldier set to guard an army, camp, or other place, from surprise, to observe the approach of danger, and give notice of it; a sentry.