A linear function of x is a function of the form y = ax + b, where a and b are constants.  It's called a "linear" function because a plot of the x and y coordinates for which this equation is true takes the shape of a straight line.

Contents of this section: Linear Patterns Linear Regression Back to the "Basic Principles" section.

More about the pages of information about linear functions and linear equations.

Patterns -- given a table of x and y values that are linearly related, how can you spot the formula that relates them?
Linear Regression -- how to calculate the linear regression as a descriptive statistic