Line (Geometry)

In geometry, a straight line, usually just called a line, is an infinitely long object with no width, depth or curvature, a special case of a curve that idealizes physical objects such as a taut string, a straightedge or a ray of light. Lines are one-dimensional and may be embedded in spaces of two, three or more dimensions; in everyday language the word line can also mean a line segment, the part of a line bounded by two endpoints. Euclid's Elements defined a straight line as a breadthless length that lies evenly with the points on itself, and introduced several postulates as basic unprovable properties on which the rest of geometry was built; the terms Euclidean line and Euclidean geometry were introduced later to distinguish this classical notion from the non-Euclidean, projective and affine geometries developed since the end of the nineteenth century.