Pe*num"bra (?), n. [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.
⇒ 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.
© Webster 1913.