sentinel

n. [ F. sentinelle (cf. It. sentinella); probably originally, a litle path, the sentinel's beat, and a dim. of a word meaning, path; cf. F. sente path. L. semita; and OF. sentine, sentele, senteret, diminutive words. Cf. Sentry. ] 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. [ 1913 Webster ]

The sentinels who paced the ramparts. Macaulay. [ 1913 Webster ]

2. Watch; guard. [ Obs. ] “That princes do keep due sentinel.” Bacon. [ 1913 Webster ]

3. (Zool.) A marine crab (Podophthalmus vigil) native of the Indian Ocean, remarkable for the great length of its eyestalks; -- called also sentinel crab. [ 1913 Webster ]