A flat two dimensional surface is called a plane.
A plane is a surface but it is flat (no curvature). It has length in one direction and width in second direction. Thus, it is a two dimensional surface. It can or cannot have thickness and this factor is not considered to determine a plane geometrically.
It seems the surface is a plane from top view but we cannot tell this is a plane. So, examine it from side view to check the curvature of the surface.
It is a straight line from side view. It means it is a flat surface. Therefore, this surface is known as a plane geometrically.
A plane has three properties.