Line (geometry)

In Euclidean geometry, a line is a straight curve. When geometry is used to model the real world, lines are used to represent straight objects with negligible width and height. Lines are an idealization of such objects and have no width or height at all and are usually considered to be infinitely long.
In geometry, a line is usually a primitive notion of an axiom system. It consists of a set of points and is a subset of a plane. In differential geometry, the concept of a line is generalized to a geodesic. In synthetic geometry a line is one of the primitive notions in the systems of Euclid, Karl von Staudt, and David Hilbert. A line is a defined term in the systems of Giuseppe Peano, Mario Pieri and Alessandro Padoa.
A line segment is a part of a line that is bounded by two distinct end points and contains every point on the line between its end points. Depending on how the line segment is defined, either of the two end points may or may not be part of the line segment. Two or more line segments may have some of the same relationships as lines, such as being parallel, intersecting, or skew.


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