I have this definition of quadrilateral in my notes:
Suppose we have a real affine space. We consider four points $A,B,C,D$ forms a quadrilateral if this points are included in the same plane and if we choose any three of them, they are not included in the same line. The points $A,B,C,D$ are the quadrilateral vertices, segments $AB, BC, CD, DA$ are the quadrilateral sides and segments $AC, BD$ are the quadrilateral diagonals.
Following the definition, I started to do some sketches and I found this strange situation:
NOTE: The points $A,B,C,D$ in the picture satisfies all the requirements of the definition. Then, we can say $AB, BC, CD, DA$ are the sides (painting in red) and $AC, BD$ are the diagonals (painting in pink)
But there is something strange about this picture. This is not the conventional idea that I have in my mind of a quadrilateral because there are two different regions!!
My question is the following:
Are there any mistakes in my definition of quadrilateral? Or maybe this picture represents a pathological type of quadrilateral that I don't know nothing about it?