1. A partial shadow, as in an eclipse, between regions of complete shadow and complete illumination.

2. The diffuse outer part of a sunspot.

3. An area in which something exists to a lesser or uncertain degree:"The First Amendment has a penumbra where privacy is protected from governmental intrusion"(Joseph A. Califano, Jr.)

4. An outlying surrounding region; a periphery:"Downtown Chicago and its penumbra also stand rejuvenated"(John McCormick).