What does "penumbra" mean?

Penumbra: (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.