How do you spell Penumbra

Available Definitions:
1)  n. - An incomplete or partial shadow.
2)  n. - 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.
3)  n. - The part of a picture where the shade imperceptibly blends with the light.