A function whose graph forms a straight line is called a linear function .

A linear equation is any equation that can be written in the standard form shown below.

The y-intercept is the y-coordinate of the point where the graph intersects the y-axis. The x-coordinate of this point is always 0.

The x-intercept is the x-coordinate of the point where the graph intersects the x-axis. The y-coordinate of this point is always 0.

A direct variation is a special type of linear relationship that can be written in the form y = kx, where k is a nonzero constant called the constant of variation .

A family of functions is a set of functions whose graphs have basic characteristics in common. For example, all linear functions form a family because all of their graphs are the same basic shape.

A parent function is the most basic function in a family. For linear functions, the parent function is f (x) = x.

The graphs of all other linear functions are transformations of the graph of the parent function, f (x) = x. A transformation is a change in position or size of a figure. There are three types of transformations—translations, rotations, and reflections.