boundary

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Something that indicates a border or limit.
2. n. The border or limit so indicated.

Century Dictionary and Cyclopedia

1. n. 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.
2. n. 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.

Wiktionary

1. n. The dividing line or location between two areas.
2. n. cricket An edge or line marking an edge of the playing field.
3. n. cricket An event whereby the ball is struck and either touches or passes over a boundary (with or without bouncing), usually resulting in an award of 4 (a four) or 6 (a six) runs respectively for the batting team.
4. n. topology (of a set) the set of points in the closure of a set , not belonging to the interior of that set.

GNU Webster's 1913

1. n. 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.

WordNet 3.0

1. n. a line determining the limits of an area
2. n. the greatest possible degree of something
3. n. the line or plane indicating the limit or extent of something