Editing Line (geometry) (section)

{{Short description|Straight figure with zero width and depth}}
{{Use British English|date=August 2022}}
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{{about||the graphical concept|Line (graphics)}}
[[File:Gerade.svg|alt=see caption|thumb|A red line near the [[Origin (mathematics)|origin]] on the two-dimensional [[Cartesian coordinate system]]]]
{{General geometry |1d}}

In [[geometry]], a '''straight line''', usually abbreviated '''line''', is an infinitely long object with no width, depth, or [[curvature]], an idealization of such physical objects as a [[straightedge]], a taut string, or a [[ray (optics)|ray of light]]. Lines are [[space (mathematics)|space]]s of [[dimension]] one, which may be [[Embedding|embedded]] in spaces of dimension two, three, or higher. The word ''line'' may also refer, in everyday life, to a [[line segment]], which is a part of a line delimited by two [[Point (geometry)|points]] (its ''endpoints'').

[[Euclid's Elements|Euclid's ''Elements'']] defines a straight line as a "breadthless length" that "lies evenly with respect to the points on itself", and introduced several [[postulate]]s as basic unprovable properties on which the rest of geometry was established. '''''Euclidean line''''' and ''[[Euclidean geometry]]'' are terms introduced to avoid confusion with generalizations introduced since the end of the 19th century, such as [[Non-Euclidean geometry|non-Euclidean]], [[Projective geometry|projective]], and [[affine geometry]].