A relation, or relationship, is a rule that relates the elements of one set to the elements of another set. The first set is called the set of inputs and the second set is called the set of outputs. In a relation, an input can have zero outputs, one output, or more than one output. Often, a relation is thought of as a set of ordered pairs of the form $(x,y).$ In this case, the $x\text{-}$values represent the inputs and the $y\text{-}$values the outputs. However, a relation can also be represented using a mapping diagram, a table of values, or a set of points on a coordinate plane.