boundary

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Something that indicates a border or limit.
• noun The border or limit so indicated.

from The Century Dictionary.

• 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.

from the GNU version of the Collaborative International Dictionary of English.

• noun 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The dividing line or location between two areas.
• noun cricket An edge or line marking an edge of the playing field.
• noun 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.
• noun topology (of a set) the set of points in the closure of a set , not belonging to the interior of that set.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a line determining the limits of an area
• noun the greatest possible degree of something
• noun the line or plane indicating the limit or extent of something