The Collaborative International Dictionary
Sentinel \Sen"ti*nel\, 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.]
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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. Watch; guard. [Obs.] ``That princes do keep due sentinel.''
--Bacon.(Zo["o]l.) A marine crab ( Podophthalmus vigil) native of the Indian Ocean, remarkable for the great length of its eyestalks; -- called also sentinel crab.