Given a pair of related measures (X and Y) on each of a set of items, the term "regression" is used to characterize the manner in which one of the measures (for example the Y measures) change as the other measure (in this case, the X measure) changes. For any set of related measures, it is possible to specify a line that approximates the mean of the Y measures for those items with a given X measure. By revealing how the mean of the Y measures change as the various X measures change, this line is understood to describe the regression of Y on X.

It is noteworthy that for the same set of related measures there is always a second regression line that describes the regression of X on Y.

The regression line is the predicted value of Y for each value of X.