Vertex

A vertex is a corner. More than one vertex are called vertices. A vertex is a point where two or more lines, curves, rays, or sides meet, and is often represented by letters such as ${\displaystyle P}$, ${\displaystyle Q}$, ${\displaystyle R}$ and ${\displaystyle S}$.[1]

B is an ear because the line between C and D is completely inside the shape. C is a mouth because the line between A and B is completely outside the shape.

For example, the vertex of an angle is the point where the two edges of the angle intersect, and a vertex of a cube is simply one of its corners—of which there are 8.[2]

The concept of vertex applies to both two-dimensional and three-dimensional geometrical objects. For example, a tetrahedron has 4 vertices, and a pentagon has 5 vertices.[3]