From The Collaborative International Dictionary of English v.0.44 [gcide]:
Penumbra \Pe*num"bra\, noun [NL., fr. L. paene almost + umbra shade.]
1. An incomplete or partial shadow.
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.
Note: The faint shade surrounding the dark central portion of a solar spot is also called the {penumbra}, and sometimes {umbra}.
3. (Paint.) The part of a picture where the shade imperceptibly blends with the light.
From WordNet (r) 2.0 [wn]:
noun
1: a fringe region of partial shadow around an umbra [also: {penumbrae} (pl)]
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