A **line** is a one-dimensional infinite set of points that extend into two directions without curvature.

It may also be seen as a "double-sided" ray.

In coordinate geometry, a line is defined in the Cartesian coordinate plane as the infinite set of points where each point satisfies a given equation in the form: , where A, B and C are constants.

Furthermore, the equation of a 'line' (parallel or perpendicular) can be found from a graph in which equation is equal to where denotes the gradient, and denotes the intercept.