Penumbra
Penumbra , noun
[New Latin, from Latin paene almost + umbra shade.]
1.
An incomplete or partial shadow.
2.
(Astronomy) The shadow cast, in an eclipse, where the light is partly, but not wholly, cut off by the intervening body; the space of partial illumination between the umbra, or perfect shadow, on all sides, and the full light. — Sir I. Newton
The faint shade surrounding the dark central portion of a solar spot is also called the penumbra, and sometimes umbra.
3.
(Painting) The part of a picture where the shade imperceptibly blends with the light.