Astronomy. 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). the grayish marginal portion of a sunspot. Compare umbra (def 3b).
a shadowy, indefinite, or marginal area.
a fringe region of half shadow resulting from the partial obstruction of light by an opaque object
(astronomy) the lighter and outer region of a sunspot
(painting) the point or area in which light and shade blend