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  

/ 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

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

There was, in fact, very little discernible boundary in my grandmother’s house between breakfast, snack and dessert.

From Salon ● Aug. 31, 2026

Searches for the international boundary water in Google Maps now direct U.S. users to a renamed “Lake America.”

From Los Angeles Times ● Aug. 30, 2026

The United States Tennis Association, which organises the event, says it works with the local police and parks department to reduce smoking on the site boundary.

From BBC ● Aug. 29, 2026

Determining that boundary is one of the next major goals.

From Science Daily ● Aug. 23, 2026

The largest of the islands, Honshu, is in an especially precarious position, straddling the boundary between the Eurasian and North American Plates.

From "Meltdown" by Deirdre Langeland

But if you’re not speaking up and setting boundaries, her assumption that your offer included dinner is just as reasonable as your assumption that it didn’t.

From MarketWatch ● Sep. 2, 2026

"Doing this puts boundaries around homework and makes it easier to ring-fence distraction-free time," she says.

From BBC ● Aug. 30, 2026

Managers are adjusting to a world where buying the first round and cracking a joke could be seen as keeping the mood light—or blurring boundaries.

From The Wall Street Journal ● Aug. 28, 2026

Even amid the era’s accelerating glam rock movement, “Rocky Horror” pushed the boundaries of mainstream pop culture as an outré musical about a mad scientist with a theme song called “Sweet Transvestite.”

From Los Angeles Times ● Aug. 26, 2026

In theory, they would chat, make their apologies, agree to respect the boundaries of Fifth Harbor, then all head out to find a drink together—at least that’s what Per Haskell had insisted.

From "Six of Crows" by Leigh Bardugo

Related Words

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