penumbra

Concise Oxford English Dictionary © 2008 Oxford University Press:
penumbra /pɪˈnʌmbrə/

▶noun (pl. penumbrae /-briː/ or penumbras)

• 1 the partially shaded outer region of the shadow cast by an opaque object, especially the area of the earth or moon experiencing a partial eclipse.

• 2 Astronomy the less dark outer part of a sunspot.

– derivatives
penumbral adjective.

– origin C17: mod. L., from L. paene ‘almost’ + umbra ‘shadow’.