boundary

American  

[baun-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  

/ ˈbaʊndərɪ, -drɪ /

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

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Etymology

Origin of boundary

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

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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

For their part, they were probably embarrassed to realize that you had given enough and erected an invisible, though unmistakable, boundary between the two households.

From MarketWatch ● Oct. 25, 2026

Known as the Fourier limit, this boundary is determined by the time required for a quantum emitter to release its energy into its surroundings.

From Science Daily ● Oct. 20, 2026

Wright also interrogates the boundary between severe mental illness and evil, and wonders whether, even in extreme cases, redemption might be possible.

From Los Angeles Times ● Oct. 20, 2026

Also, the currency pair has risen clearly above the upper boundary of the daily Ichimoku cloud, but the top of weekly cloud capped its gains at 1.2871 this week, the strategist says.

From The Wall Street Journal ● Oct. 16, 2026

The western boundary of the Black Belt met the eastern boundary of Packingtown at Wentworth Avenue, presenting the opportunity for all sorts of cooperative interaction between the races: in stores, schools, parks, each other’s homes.

From "A Few Red Drops: The Chicago Race Riot of 1919" by Claire Hartfield

Even more surprising were the boundaries separating these regions, which were hundreds of times wider than expected.

From Science Daily ● Oct. 25, 2026

Once faith hangs in the balance, Fielder can navigate the viewer and Seyfried through its opaque boundaries, taking all of us to Evans’ house for an uncomfortably intimate dinner.

From Salon ● Oct. 25, 2026

This decadent confection—pistachio, raspberry and fudge brownie, we’re told—is an apt conclusion to a show that’s a treat for anyone interested in the surprising ways artists continue to push the boundaries of well-known materials.

From The Wall Street Journal ● Oct. 23, 2026

Jiang Defu, CEO of the production company Bona Film Group, acknowledged the lukewarm reception in a recent interview but said the film was pushing boundaries.

From Barron's ● Oct. 23, 2026

It’s my anger that allows him to slither between my boundaries, allows him to keep laying the blame of our non-relationship at my feet.

From "Red Flags and Butterflies" by Sheryl Azzam

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Related Words

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

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