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Reference.org
Line (geometry)
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<p>In <a href="/facts/Geometry/wTQf5ffQ">geometry</a>, a straight line, usually abbreviated line, is an infinitely long object with no width, depth, or <a href="/facts/Curvature/RFCAEsj8">curvature</a>, an idealization of such physical objects as a <a href="/facts/Straightedge/6o9Xp9Eb">straightedge</a>, a taut string, or a <a href="/facts/Ray_(optics)/PgUEC7Sm">ray of light</a>. Lines are <a href="/facts/Space_(mathematics)/G6rt4bQ8">spaces</a> of <a href="/facts/Dimension/0ktmUyac">dimension</a> one, which may be <a href="/facts/Embedding/HKeVKc42">embedded</a> in spaces of dimension two, three, or higher. The word <i>line</i> may also refer, in everyday life, to a <a href="/facts/Line_segment/V8E0M4qk">line segment</a>, which is a part of a line delimited by two <a href="/facts/Point_(geometry)/6YPAvX2a">points</a> (its <i>endpoints</i>). </p><p><a href="/facts/Euclid%27s_Elements/nPKvCIxs">Euclid's <i>Elements</i></a> defines a straight line as a "breadthless length" that "lies evenly with respect to the points on itself", and introduced several <a href="/facts/Postulate/mLjP14Mc">postulates</a> as basic unprovable properties on which the rest of geometry was established. <i>Euclidean line</i> and <i><a href="/facts/Euclidean_geometry/V6cTcoCy">Euclidean geometry</a></i> are terms introduced to avoid confusion with generalizations introduced since the end of the 19th century, such as <a href="/facts/Non-Euclidean_geometry/efKa7De2">non-Euclidean</a>, <a href="/facts/Projective_geometry/rHKSaEbx">projective</a>, and <a href="/facts/Affine_geometry/yUGgTx2n">affine geometry</a>. </p>