Final answer:
A curvilinear relationship exists when the relationship between two variables is not a straight line, but instead follows a curve. Examples include quadratic and exponential relationships.
Step-by-step explanation:
A curvilinear relationship exists when the relationship between two variables is not a straight line, but instead follows a curve. One example of a curvilinear relationship is a quadratic relationship, where the relationship between the variables is represented by a curve that forms a U-shape or an inverted U-shape. Another example is an exponential relationship, where the relationship between the variables is represented by a curve that either increases or decreases at an increasingly faster rate.