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From The Collaborative International Dictionary of English v.0.48:
Penumbra \Pe*num"bra\, 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] Note: 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]