Function defined on the set of real numbers by a relationship of the form y = kx where k is a non-zero real number.
The graph of a direct variation function always passes through the origin (0, 0) of the Cartesian plane.
Example
This is the graph of the function f defined by the rule f(x) = -2x.
The rate of change of this function is −2.
Since the axes of the Cartesian plane are orthogonal, we can say that the slope is –2.