Sentinel

Part of speech: noun (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." Bacon.

3. A marine crab (Podophthalmus vigil) native of the Indian Ocean, remarkable for the great length of its eyestalks; -- called also sentinel crab.

Part of speech: verb (v.)

1. To watch over like a sentinel. "To sentinel enchanted land." [R.] Sir W. Scott.

2. To furnish with a sentinel; to place under the guard of a sentinel or sentinels.