In algebra, there is frequently more than a single name for something. That's true of ordered pairs. The horizontal (or *x*) coordinates of a set of ordered pairs constitute the domain of a relation, while the range is determined by the vertical (or *y*) coordinates. A relation in which none of the domain numbers appears more than once is called a “function.” Following are definitions with which you should be familiar as you work with relations and functions.

#### Relation

A **relation** is a set of ordered pairs that can be represented by a diagram, graph, or sentence. Figure 1 shows several examples of relations.