Source : Webster's Revised Unabridged Dictionary (1913)

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.

            The sentinels who paced the ramparts. --Macaulay.

   2. Watch; guard. [Obs.] ``That princes do keep due
      sentinel.'' --Bacon.

   3. (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}.