=boundary

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.

But still his native country lies
Beyond the boundaries of the skies. --N. Cotton.

That bright and tranquil stream, the boundary of Louth
and Meath. --Macaulay.

Sensation and reflection are the boundaries of our
thoughts. --Locke.

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.