Definition:Real Function/Definition by Formula

Definition
A function $f: S \to T$ can be considered as a formula which tells us how to determine what the value of $y \in T$ is when we have selected a value for $x \in S$.

Also known as
Some sources, possibly in an attempt to improve the accessibility of the subject, refer to the formula for a function as a recipe.

Other sources use the term rule.

Also see

 * Definition:Real-Valued Function