Penumbra

Part of speech: noun (n.)

1. An incomplete or partial shadow.

2. 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.

3. The part of a picture where the shade imperceptibly blends with the light.