Pronunciation: (pi-num'bru),[key] —n., —pl.-braePronunciation: (-brē),[key]-bras. 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.