The outer incomplete shadow formed during an eclipse is called the penumbra.

Here's a breakdown:

* Eclipse: An eclipse occurs when one celestial body passes in front of another, blocking its light.

* Umbra: The darkest part of the shadow, where the light source is completely blocked. During a total solar eclipse, the umbra is where the sun is completely obscured.

* Penumbra: The lighter, outer part of the shadow where the light source is only partially blocked. During a partial solar eclipse, the penumbra is where the sun appears partially obscured.

So, the penumbra is the region of partial shadow during an eclipse.