noun.
That which serves to indicate the bounds or limits of anything; hence, a limiting or bounding line; a bound: as, the horizon is the boundary of vision; the northern boundary of the United States.
noun.
Synonyms Boundary, Bound, Border, Confine, Frontier. A boundary, in its stricter sense, is a visible mark indicating a dividing-line between two things, or it is that line itself; it marks off a given thing from other things like in kind, as one field or country from another. A bound, on the other hand, is the limit or furthest point of extension of one given thing, that which limits it not being specially considered; it can be used of that which is not limited by anything like in kind: as, the boundaries of a field, but the bounds of space; the boundaries of a science, but the bounds of knowledge. Hence the figurative uses of bound: as, “I believe I speak within bounds,” where boundaries would be absurd. Thus, the bounds of a parish may be defined by certain marks or boundaries, as heaps of stones, dikes, hedges, streams, etc., separating it from the adjoining parishes. But the two words are often interchangeable. A border is a belt or band of territory lying along a bound or boundary. A confine is the region at or near the edge, and generally a narrower margin than a border. A frontier is a border viewed as a front or place of entrance: as, he was met at the frontier. The word is used most in connection with military operations: as, their frontiers were well protected by fortresses.