Source : Webster's Revised Unabridged Dictionary (1913)

Sentry \Sen"try\, n.; pl. {Sentires}. [Probably from OF.
   senteret a little patch; cf. F. sentier path, and OF. sente.
   See {Sentinel}.]
   1. (Mil.) A soldier placed on guard; a sentinel.

   2. Guard; watch, as by a sentinel.

            Here toils, and death, and death's half-brother,
            sleep, Forms terrible to view, their sentry keep.
                                                  --Dryden.

   {Sentry box}, a small house or box to cover a sentinel at his
      post, and shelter him from the weather.

Source : WordNet®

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