Sen·ti·nel, n.
1. 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.
The sentinels who paced the ramparts. --Macaulay.
2. Watch; guard. [Obs.] “That princes do keep due sentinel.”
3. Zool. A marine crab (Podophthalmus vigil) native of the Indian Ocean, remarkable for the great length of its eyestalks; -- called also sentinel crab.