Definition:Polygon/Opposite

Definition
When a polygon has an even number of sides, each side has an opposite side, and each vertex likewise has an opposite vertex.

When a polygon has an odd number of sides, each side has an opposite vertex.

The opposite side (or opposite vertex) to a given side (or vertex) is that side (or vertex) which has the same number of sides between it and the side (or vertex) in question.

Also known as
In the same way that vertices can be called angles, an opposite vertex is often referred to as an opposite angle.