Definition: As shown in the image, the two parallel lines are both intersected by another line. The alternate angle 1 and angle 4 will share the same measure of degrees. Likewise angle 2 and angle 3 are alternate angles and share the same measure of degrees. The alternate angles in the image are both interior angles. The same will hold true for exterior angles. Alternate angles are equal.
Examples:In geometry, two angles that lie on opposite sides and at opposite ends of a line that goes through two or more lines on the same plane (transversal). The alternate angles formed by the transversal of the two parallel lines will be equal.