The Collaborative International Dictionary of English v.0.48:

Boundary \Bound"a*ry\, n.; pl. Boundaries [From Bound a
   limit; cf. LL. bonnarium piece of land with fixed limits.]
   That which indicates or fixes a limit or extent, or marks a
   bound, as of a territory; a bounding or separating line; a
   real or imaginary limit.
   [1913 Webster]

         But still his native country lies
         Beyond the boundaries of the skies.      --N. Cotton.
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         That bright and tranquil stream, the boundary of Louth
         and Meath.                               --Macaulay.
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         Sensation and reflection are the boundaries of our
         thoughts.                                --Locke.
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   Syn: Limit; bound; border; term; termination; barrier; verge;
        confines; precinct.

   Usage: Bound, Boundary. Boundary, in its original and
          strictest sense, is a visible object or mark
          indicating a limit. Bound is the limit itself. But in
          ordinary usage the two words are made interchangeable.
          [1913 Webster]