penumbra

n. [ NL., fr. L. paene almost + umbra shade. ] 1. An incomplete or partial shadow. [ 1913 Webster ]

2. (Astron.) 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. [ 1913 Webster ]

☞ The faint shade surrounding the dark central portion of a solar spot is also called the penumbra, and sometimes umbra. [ 1913 Webster ]

3. (Paint.) The part of a picture where the shade imperceptibly blends with the light. [ 1913 Webster ]