The Collaborative International Dictionary of English v.0.48:

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