Pronunciation: (pi-num'bru), [key]
—pl. -braePronunciation: (-brē), [key] -bras.
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.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.