A linear function is any function that graphs to a straight line. What this means mathematically is that the function has either one or two variables with no exponents or powers. If the function has more variables, the variables must be constants or known variables for the function to remain a linear function.
The easiest way to define a nonlinear function is as a function that is not a linear function. ... Graphically, a linear function is a function whose graph is a line. Algebraically, a linear function can be defined as a polynomial with highest exponent equal to 1 or a horizontal line (y = c where c is a constant).