The graph of y = ax + b, the prototypical linear relationship.
A line is the shortest distance between any two points in Euclidean space. A line is implicitly a straight line. It has only one dimension, length, may be infinite in extent, and is the shortest distance between any two points.
Although in common parlance a straight line is not curved, it is a curve in the mathematical sense (i.e., a mapping of an infinite set of points into a space). Mathematically, a line may be determined by the presence of any two points in an n-dimensional space (where n is two or more). A line segment is a piece of a line with definite endpoints.
In two dimensional Cartesian coordinates, the equation of a line has the form y = ax + b, where a and b are constants.