**One-to-One Function**

A function is a **one-to-one function** if every element or y-value in the range comes from a unique element or x-value in the domain. In other words, a function is a one-to-one function if it never maps or associates two or more x-values to the same y-value.

**Example:**

Here is a more “advanced” definition:

A function is a** one-to-one function** if an element in the codomain has a pre-image, then the pre-image must be unique.

Example: