What is the name of the area of space into which the light from the source does not fall?

Such an area of any space in which there is no light and no illumination from a light source that shines somewhere is called a shadow.

Typically, a shadow, or completely dark space, is located directly behind the object to which the light is directed.

There is also a penumbra region. In this area there is light from the source, but it is not enough. This is the transition area between the shadow area and the lighted area.

In physics, the illumination of a terrain is defined using the letter E. The higher it is, the better the terrain is illuminated.