penumbra

noun (plural penumbrae /-briː/ or penumbras)

• 1the partially shaded outer region of the shadow cast by an opaque object.
• Astronomy the shadow cast by the earth or moon over an area experiencing a partial eclipse.
• Astronomy the less dark outer part of a sunspot, surrounding the core.

• 2a peripheral or indeterminate area or group:an immense penumbra of theory surrounds any observation