A **function** is a relationship between two quantities: for every value of the independent variable (usually *x*), there is exactly one value of the dependent variable (usually *y*). Functions can be thought of as a process: when a value is put in, an action (or operation) is performed and a different value comes out.

In a function, each input has *exactly one* output. Graphically this means the graph passes the **vertical line test**: anywhere a vertical line is drawn on the graph, the line intersects the curve at exactly one point.