boundary

American  

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

noun

plural

boundaries

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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Related Words

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

• transboundary adjective

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

Drag was an important part of Ortiz’s approach to playing Jim Jimenez, the boundary breaking nonbinary pirate heartthrob on HBO’s cult show “Our Flag Means Death.”

From Los Angeles Times • Apr. 1, 2026

This area is separated from the rest of the cell by an actin-myosin condensate barrier, which acts like a physical boundary and directs proteins to the advancing edge.

From Science Daily • Apr. 1, 2026

Unless the stock can get firmly above the upper boundary of the resistance zone, which comes in around $298, the key downside level to watch is the March 12 intraday low of around $279.

From MarketWatch • Mar. 31, 2026

"These days Hong Kong is their best hope to attract global investors and position themselves as a player not fully constrained by the boundary of the mainland market," she adds.

From BBC • Mar. 29, 2026

In an instant I am desperately afraid because I have crossed a boundary.

From "The Adoration of Jenna Fox" by Mary E. Pearson

Related Words

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