It is the whole system formed by two homogeneous and transparent means.
When this separation takes place on a flat plane, we then call it a flat plane.
The figure below represents a flat diopter, in the separation between water and air, which are two homogeneous and transparent means.
Imaging through a diopter
Consider a fisherman who sees a fish in a lake. The fish is at a depth H of the water surface. The fisherman sees him at a depth H. As shown in the figure below:
The formula that determines these distances is: