1. One who watches or guards; specifically, 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. That princes do keep due sentinel.
3. (Science: zoology) A marine crab (Podophthalmus vigil) native of the indian ocean, remarkable for the great length of its eyestalks.
Synonym: sentinel crab.
Origin: 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.