boundary

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
  1. noun
    the line or plane indicating the limit or extent of something
    synonyms: bound, bounds
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    examples:
    Rubicon
    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
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    hairline
    the natural margin formed by hair on the head
    frontier
    an international boundary or the area (often fortified) immediately inside the boundary
    heliopause
    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
    end
    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
    surface
    the extended two-dimensional outer boundary of a three-dimensional object
    shoreline
    a boundary line between land and water
    edge
    the outside limit of an object or area or surface; a place farthest away from the center of something
    interface
    (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
    brink
    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
    end
    the surface at either extremity of a three-dimensional object
    inside, interior
    the inner or enclosed surface of something
    limb
    (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
    coastline
    the outline of a coast
    silhouette
    an outline of a solid object (as cast by its shadow)
    exterior, outside
    the outer side or surface of something
    skyline
    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
    photosphere
    the intensely luminous surface of a star (especially the sun)
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    extremity
    the outermost or farthest region or point
  2. noun
    a line determining the limits of an area
    synonyms: bound, edge
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    rim
    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
    thalweg
    the middle of the chief navigable channel of a waterway that forms the boundary line between states
    lip
    either the outer margin or the inner margin of the aperture of a gastropod's shell
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    line
    a length (straight or curved) without breadth or thickness; the trace of a moving point
  3. noun
    the greatest possible degree of something
    synonyms: bound, limit
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    knife-edge
    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, cusp, verge
    the limit beyond which something happens or changes
    precipice
    the brink of a dangerous or potentially disastrous situation
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    extent
    the distance or area or volume over which something extends
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