Question

Penumbra (definition)

Answer 1

The partially shaded outer region of the shadow cast by an opaque object

or,

The shadow cast by the earth or moon over an area experiencing a partial eclipse.

or,

The less dark outer part of a sunspot, surrounding the dark core.

(The last two are about astronomy which is the branch of science which deals with celestial objects, space, and the physical universe as a whole.)