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.
   [1913 Webster]

         That bright and tranquil stream, the boundary of Louth
         and Meath.                               --Macaulay.
   [1913 Webster]

         Sensation and reflection are the boundaries of our
         thoughts.                                --Locke.
   [1913 Webster]

   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]

WordNet (r) 3.0 (2006):

boundary
    n 1: the line or plane indicating the limit or extent of
         something [syn: boundary, bound, bounds]
    2: a line determining the limits of an area [syn: boundary,
       edge, bound]
    3: the greatest possible degree of something; "what he did was
       beyond the bounds of acceptable behavior"; "to the limit of
       his ability" [syn: limit, bound, boundary]

The Devil's Dictionary (1881-1906):

BOUNDARY, n.  In political geography, an imaginary line between two
nations, separating the imaginary rights of one from the imaginary
rights of the other.