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}.

Sentinel \Sen"ti*nel\, v. t. [imp. & p. p. {Sentineled}or
   {Sentinelled}; p. pr. & vb. n. {Sentineling} or
   {Sentinelling}.]
   1. To watch over like a sentinel. ``To sentinel enchanted
      land.'' [R.] --Sir W. Scott.

   2. To furnish with a sentinel; to place under the guard of a
      sentinel or sentinels.

Source : WordNet®

sentinel
     n : a person employed to watch for something to happen [syn: {lookout},
          {lookout man}, {sentry}, {watch}, {spotter}, {scout}, {picket}]