penumbra

pe·num·bra [pi-nuhm-bruh]

1.	Astronomy. a. the partial or imperfect shadow outside the complete shadow of an opaque body, as a planet, where the light from the source of illumination is only partly cut off. Compare umbra (def. 3a). b. the grayish marginal portion of a sunspot. Compare umbra (def. 3b).

2.	a shadowy, indefinite, or marginal area.

from dictionary.com