Definition:Polygon/Internal Angle

Definition
The internal angle of a vertex of a polygon is the size of the angle between the sides adjacent to that vertex, as measured inside the polygon.

Also known as
An internal angle is also known as an interior angle.

The vertices and the sizes of the internal angles of those vertices are frequently referred to by the same letter.

Thus the angle of vertex $A$ is called angle $A$ and denoted $\angle A$.

This is considered by some to be an abuse of notation but its convenience outweighs its disadvantages.

Also see

 * Definition:External Angle