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]

WordNet (r) 3.0 (2006):

penumbra
    n 1: a fringe region of partial shadow around an umbra