boundary
Americannoun
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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.
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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.
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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.
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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.
noun
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something that indicates the farthest limit, as of an area; border
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cricket
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the marked limit of the playing area
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a stroke that hits the ball beyond this limit
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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
Inflected Forms
Nouns
Etymology
Origin of boundary
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.
Vocabulary lists containing boundary
Human Geography - Middle School
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Human Geography - High School
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Political Geography - Middle School
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Example Sentences
Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.
See Examples For:
After studying classics at Cambridge, his acting career led to the Marvel Cinematic Universe and the role of Loki, the god of mischief and “the boundary crosser.”
From Los Angeles Times ● Jul. 21, 2026
A vertical beam creates a circular boundary of light on the ground.
From Science Daily ● Jul. 6, 2026
Dani Gibson, Freya Kemp and Linsey Smith – three fielders who often share the job of patrolling the boundary – practise their high catches or sprints to retrieve and return the ball.
From BBC ● Jul. 3, 2026
If two hours of location information qualifies as a search, where exactly does the constitutional boundary lie?
From Slate ● Jun. 29, 2026
Miss Grybas explained that the Urals were the boundary between Europe and Asia.
From "Between Shades of Gray" by Ruta Sepetys
Albanians have been protesting against the project for months, but it was when barbed wire appeared six weeks ago to mark out the boundaries of the complex that anger erupted.
From Barron's ● Jul. 23, 2026
He added testing technology is evolving quickly but, where riders and teams push the boundaries, doping is always going to be a risk factor.
From BBC ● Jul. 22, 2026
As words such as trauma, boundaries and narcissism have entered the mainstream, they are more misused and diluted.
From The Wall Street Journal ● Jul. 19, 2026
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
I could hear my breathing, could feel the rise and fall of my chest, gases seeping across nothing boundaries.
From "Anthem of a Reluctant Prophet" by Joanne Proulx
Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023
Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.