(Terms of physics/lighting)
The shadow volume behind an object lit by an area light source (in contrast to a point light source) doesn't have sharp boundaries. This is caused by the fact that each point in the boundary area is only partially shadowed. The area (volume) in full shadow is the umbra, the boundary area the penumbra.
(The graphic is actually inaccurate, since the penumbra doesn't have a constant shadow density, but rather a gradient ranging from shadow to light)