sen·ti·nel /ˈsɛntnəl, ˈsɛntṇəl/
  哨兵(vt.)警戒,守衛
  sen·ti·nel /ˈsɛntṇəl/ 形容詞
  sentinel
  標記; 始終標記
  sentinel
  哨符
  Sen·ti·nel, n.
  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.”
  3. Zool. A marine crab (Podophthalmus vigil) native of the Indian Ocean, remarkable for the great length of its eyestalks; -- called also sentinel crab.
  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.]
  2. To furnish with a sentinel; to place under the guard of a sentinel or sentinels.
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  sentinel
       n : a person employed to watch for something to happen [syn: lookout,
            lookout man, sentry, watch, spotter, scout, picket]