boundary

UK: /ˈbaʊndəri/ US: /ˈbaʊndəri, -dri/ ,(boun′də rē, -drē)

Inflections of 'boundary' (n): npl: boundaries

WordReference Random House Learner's Dictionary of American English © 2025

bound•a•ry /ˈbaʊndəri, -dri/   n. [countable] , pl. -ries. 

1. something that indicates bounds or limits, as a line:
   A mountain range forms a natural boundary between the two countries.

Synonyms

boundary, border, frontier refer to something that divides one territory, state, country, etc., from another. boundary most often refers to a line on a map; it may be a physical feature, such as a river: Boundaries on this map are shown in red. border refers to a political or geographic dividing line; it may also refer to the region next to the actual line: crossing the Mexican border. frontier refers specifically to a border between two countries or the region adjoining this border: Soldiers guarded the frontier between Russia and China.

WordReference Random House Unabridged Dictionary of American English © 2025

bound•a•ry  (boun′də rē, -drē), n., pl. -ries. 

1. something that indicates bounds or limits;
   a limiting or bounding line.
2. Mathematics Also called frontier. the collection of all points of a given set having the property that every neighborhood of each point contains points in the set and in the complement of the set.
3. [Cricket.] a hit in which the ball reaches or crosses the boundary line of the field on one or more bounces, counting four runs for the batsman. Cf. six (def. 5).

• bound³ + -ary 1620–30

Synonyms

• 1. Boundary, border, frontier share the sense of that which divides one entity or political unit from another.
  Boundary, in reference to a country, city, state, territory, or the like, most often designates a line on a map:
  boundaries are shown in red.
  Occasionally, it also refers to a physical feature that marks the agreed-upon line separating two political units:
  The Niagara River forms part of the boundary between the United States and Canada.
  Border is more often used than boundary in direct reference to a political dividing line; it may also refer to the region (of, for instance, a country) adjoining the actual line of demarcation:
  crossing the Mexican border; border towns along the Rio Grande.
  Frontier may refer to a political dividing line:
  crossed the Spanish frontier on Tuesday.
  It may also denote or describe the portion of a country adjoining its border with another country (towns in the Polish frontier) or, especially in North America, the most remote settled or occupied parts of a country:
  the frontier towns of the Great Plains.
  Frontier, especially in the plural, also refers to the most advanced or newest activities in an area of knowledge or practice:
  the frontiers of nuclear medicine.

Collins Concise English Dictionary © HarperCollins Publishers::

boundary / ˈbaʊndərɪ -drɪ/ noun ( -ries)

1. something that indicates the farthest limit, as of an area; border
2. the marked limit of the playing area
3. a stroke that hits the ball beyond this limit
4. the four runs scored with such a stroke, or the six runs if the ball crosses the boundary without touching the ground