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

Hawking proposed that the size of a black hole's event horizon, the boundary beyond which even light cannot escape, is proportional to its entropy.

From Science Daily ● Jul. 13, 2026

Bairstow, who made 136 from 92 balls, fell just before the end but Stokes remained and thumped the winning boundary to take the home side to victory with 22 overs to spare.

From BBC ● Jul. 12, 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

Nor is the boundary where one phrase leaves off and the next one begins marked on the page^^The reader has to guess, and it’s up to the writer to ensure that the guesses are correct.

From "The Sense of Style" by Steven Pinker

The situation highlights a difficult reality for policymakers: Some problems can cross borders regardless of tariffs, political boundaries or enforcement measures.

From Salon ● Jul. 18, 2026

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

Coaches will always be looking for the marginal gain - ways to push the boundaries to score a crucial goal.

From BBC ● Jul. 8, 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

“You’ll be going to Tonks’s parents. Once you’re within the boundaries of the protective enchantments we’ve put on their house, you’ll be able to use a Portkey to the Burrow. Any questions?”

From "Harry Potter and the Deathly Hallows" by J.K. Rowling

Related Words

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