boundary

American  

[boun-duh-ree, -dree] / ˈbaʊn də ri, -dri /

noun

boundaries plural

1. a line or limit where one thing ends and another begins, or something that indicates such a line or limit.

   The ancient wall still serves as the city's outer boundary.

   These studies straddle the boundaries between computational and social sciences.

   Synonyms:

   limit, bound

2. a limit that separates acceptable behavior from unacceptable behavior.

   I'm just looking for a partner who can respect my boundaries.

   Guiding children toward responsible money habits requires setting boundaries.

3. Also called frontier.  Mathematics. 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.

4. 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.

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boundary British  

/ -drɪ, ˈbaʊndərɪ /

noun

1. something that indicates the farthest limit, as of an area; border

2. cricket

   1. the marked limit of the playing area

   2. a stroke that hits the ball beyond this limit

   3. the four runs scored with such a stroke, or the six runs if the ball crosses the boundary without touching the ground

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Synonym Usage

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.

Other Word Forms

Derived Forms

• transboundary
  adjective

Inflected Forms

Nouns

• boundaries
  plural

Etymology

Origin of boundary

First recorded in 1620–30; bound ³ + -ary

Explanation

If you play a sport you are familiar with the term "in bounds," which refers to the boundary, or limits of the playing field that the players must stay within. It is also the outer limits of any space. A boundary is a border and it can be physical, such as a fence between two properties, or abstract, such as a moral boundary that society decides it is wrong to cross. If you have no sense of boundaries, you probably annoy people sometimes by getting too close to them or talking about inappropriate topics. Gustave Flaubert once said, “Earth has its boundaries, but human stupidity is limitless.” A boundary is basically what limits us.

Example Sentences

See Examples For:

boundary boundaries

Sobers was briefly caught by Roger Davis at long-off from the fifth ball - but the fielder fell over the boundary rope in taking the catch, and six was signalled.

From BBC ● Jul. 17, 2026

The location is now a collection of cold storage facilities, parking lots and warehouses in Skid Row near its boundary with the Arts District.

From Los Angeles Times ● Jul. 1, 2026

If two hours of location information qualifies as a search, where exactly does the constitutional boundary lie?

From Slate ● Jun. 29, 2026

Grain boundary diffusion involves applying tiny quantities of the most expensive and elusive rare earths, known as heavy rare earths, to a magnet to make it heat-resistant without sacrificing magnetic strength.

From The Wall Street Journal ● Jun. 29, 2026

The horrible armies were on the point of joining battle, to dispute the imaginary boundary between their glass trays when Merlyn came to his rescue.

From "The Once and Future King" by T. H. White

Industry governance of AI is promising, but the Constitution limits how much authority Congress may vest in private actors, and the judiciary is still defining exactly where those boundaries lie.

From The Wall Street Journal ● Jul. 10, 2026

On a true pitch surrounded by short boundaries, the target looked around par, before Archer and Tongue ensured India would not get close.

From BBC ● Jul. 7, 2026

Researchers are trying to get closer to social structures, forms of exchange, boundaries between groups, the quality of alliances, ways of building a society.

From Science Daily ● Jul. 7, 2026

Her projects frequently blur the boundaries between artistic practice and civic inquiry.

From Los Angeles Times ● Jul. 7, 2026

Yeah, he needed to push boundaries every once in a while, but there were some boundaries he knew he couldn’t cross without changing everything.

From "Time Bomb" by Joelle Charbonneau

Related Words

• barrier
• border
• borderline
• confines
• edge
• frontier
• horizon
• line
• perimeter