In analytic geometry, the term linear function is sometimes used to mean a first degree polynomial function of one variable. These functions are called "linear" because they are precisely the functions whose graph in the Cartesian coordinate plane is a straight line.
Such a function can be written as
(called slope-intercept form), where and are real constants and is a real variable. The constant is often called the slope or gradient, while is the y-intercept, which gives the point of intersection between the graph of the function and the -axis. Changing makes the line steeper or shallower, while changing moves the line up or down.
Examples of functions whose graph is a line include the following:
The graphs of these are shown in the image at right.
For example, if and are represented as coordinate vectors, then the linear functions are those functions that can be expressed as
A function is a linear map if and only if . For other values of this falls in the more general class of affine maps.