# Alternate Interior Angle Definition

In mathematics, the **alternate interior angle** is defined as the **angle formed insides two parallel lines** when a **third line intersects these parallel lines**.

In addition, the angles are formed at the inner inside of a line are equivalent to the other's line exterior angle.

For a more precise understanding, check out the diagram listed below:

**Note:**

To know about acute angle, you can use our Acute Angle Definition.

In the above diagram, you can see that the interior angle formed in the first line will be equivalent to the second's line exterior angle.