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.

Definitions of boundary

n the line or plane indicating the limit or extent of something

bound, bounds
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the boundary in ancient times between Italy and Gaul; Caesar's crossing it with his army in 49 BC was an act of war
Mohorovicic discontinuity
the boundary between the Earth's crust and the underlying mantle
Green Line
the border marking the boundaries of the land that Israel won in its 1948 war of independence
Line of Control
a 450-mile line that is supposed to indicate the boundary between the part of Kashmir controlled by India and the part controlled by Pakistan
Mason and Dixon's line
the boundary between Maryland and Pennsylvania; symbolic dividing line between North and South before the American Civil War
Triple Frontier
the border area where Argentina and Brazil and Paraguay meet; an active South American center for contraband and drug trafficking and money laundering; a suspected locale for Islamic extremist groups
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the natural margin formed by hair on the head
an international boundary or the area (often fortified) immediately inside the boundary
the boundary marking the edge of the sun's influence; the boundary (roughly 100 AU from the sun) between the interplanetary medium and the interstellar medium; where the solar wind from the sun and the radiation from other stars meet
border, borderline, boundary line, delimitation, mete
a line that indicates a boundary
bourn, bourne
an archaic term for a boundary
district line
the boundary between two districts
county line
the boundary between two counties
city line
the boundary of a city
border, edge
the boundary of a surface
a boundary marking the extremities of something
demarcation, demarcation line, limit
the boundary of a specific area
lineation, outline
the line that appears to bound an object
the extended two-dimensional outer boundary of a three-dimensional object
a boundary line between land and water
the outside limit of an object or area or surface; a place farthest away from the center of something
(chemistry) a surface forming a common boundary between two things (two objects or liquids or chemical phases)
hard palate
the bony part of the roof of the mouth
palate, roof of the mouth
the upper surface of the mouth that separates the oral and nasal cavities
face, side
a surface forming part of the outside of an object
circuit, circumference
the boundary line encompassing an area or object
fence line
a boundary line created by a fence
property line
the boundary line between two pieces of property
state boundary, state line
the boundary between two states
the edge of a steep place
celestial sphere, empyrean, firmament, heavens, sphere, vault of heaven, welkin
the apparent surface of the imaginary sphere on which celestial bodies appear to be projected
city limit, city limits
the limits of the area occupied by a city or town
the surface at either extremity of a three-dimensional object
inside, interior
the inner or enclosed surface of something
(astronomy) the circumferential edge of the apparent disc of the sun or the moon or a planet
upper limit
the limit on the upper (or northernmost) side of something
lower limit
the limit on the lower (or southernmost) side of something
the outline of a coast
an outline of a solid object (as cast by its shadow)
exterior, outside
the outer side or surface of something
the outline of objects seen against the sky
substrate, substratum
a surface on which an organism grows or is attached
three-mile limit
the limit of a nation's territorial waters
wave front, wavefront
(physics) an imaginary surface joining all points in space that are reached at the same instant by a wave propagating through a medium
the intensely luminous surface of a star (especially the sun)
Type of:
the outermost or farthest region or point

n a line determining the limits of an area

bound, edge
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the shape of a raised edge of a more or less circular object
border, margin, perimeter
the boundary line or the area immediately inside the boundary
fringe, outer boundary, periphery
the outside boundary or surface of something
brink, threshold, verge
a region marking a boundary
upper bound
(mathematics) a number equal to or greater than any other number in a given set
lower bound
(mathematics) a number equal to or less than any other number in a given set
the middle of the chief navigable channel of a waterway that forms the boundary line between states
either the outer margin or the inner margin of the aperture of a gastropod's shell
Type of:
a length (straight or curved) without breadth or thickness; the trace of a moving point

n the greatest possible degree of something

bound, limit
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a narrow boundary
absoluteness, starkness, utterness
the quality of being complete or utter or extreme
heat barrier, thermal barrier
a limit to high speed flight imposed by aerodynamic heating
level best, maximum, utmost, uttermost
the greatest possible degree
brink, verge
the limit beyond which something happens or changes
Type of:
the distance or area or volume over which something extends

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