— penumbral, penumbrous , adj.
/pi num"breuh/ , n. , pl. penumbrae /-bree/ , penumbras .
1. Astron.
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. Cf. umbra (def. 3a).
b. the grayish marginal portion of a sunspot. Cf. umbra (def. 3b).
2. a shadowy, indefinite, or marginal area.
[ 1660-70; paen- PEN- + umbra shade ]