The Collaborative International Dictionary of English v.0.48:

Sentinel \Sen"ti*nel\, v. t. [imp. & p. p. Sentineledor
   Sentinelled; p. pr. & vb. n. Sentineling or
   Sentinelling.]
   1. To watch over like a sentinel. "To sentinel enchanted
      land." [R.] --Sir W. Scott.
      [1913 Webster]

   2. To furnish with a sentinel; to place under the guard of a
      sentinel or sentinels.
      [1913 Webster]

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.
      [1913 Webster]

            The sentinels who paced the ramparts. --Macaulay.
      [1913 Webster]

   2. Watch; guard. [Obs.] "That princes do keep due sentinel."
      --Bacon.
      [1913 Webster]

   3. (Zool.) A marine crab (Podophthalmus vigil) native of
      the Indian Ocean, remarkable for the great length of its
      eyestalks; -- called also sentinel crab.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sentinel
    n 1: a person employed to keep watch for some anticipated event
         [syn: lookout, lookout man, sentinel, sentry,
         watch, spotter, scout, picket]

U.S. Gazetteer Places (2000):

Sentinel, OK -- U.S. town in Oklahoma
   Population (2000):    859
   Housing Units (2000): 411
   Land area (2000):     0.614016 sq. miles (1.590293 sq. km)
   Water area (2000):    0.000000 sq. miles (0.000000 sq. km)
   Total area (2000):    0.614016 sq. miles (1.590293 sq. km)
   FIPS code:            66400
   Located within:       Oklahoma (OK), FIPS 40
   Location:             35.156659 N, 99.173829 W
   ZIP Codes (1990):     73664
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Sentinel, OK
    Sentinel