penumbra

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A partial shadow, as in an eclipse, between regions of complete shadow and complete illumination. See Synonyms at shade.
2. n. The grayish outer part of a sunspot.
3. n. An area in which something exists to a lesser or uncertain degree: "The First Amendment has a penumbra where privacy is protected from governmental intrusion” ( Joseph A. Califano, Jr.)
4. n. An outlying surrounding region; a periphery: "Downtown Chicago and its penumbra also stand rejuvenated” ( John McCormick).

Century Dictionary and Cyclopedia

1. n. The partial shadow between the full light and the total shadow caused by an opaque body intercepting a part of the light from a luminous body. All points within the penumbra are excluded from the view of some part of the luminous body, and are thus partially shaded; while all points within the umbra, or total shadow, are completely excluded from view of the luminous body. The figures represent the so called Hipparchan diagrams of a lunar and a solar eclipse. Any portion of the moon in penumbra appears slightly dimmed, the more so the nearer it is to the umbra. At a station of the earth in the moon's penumbra, the disk of the sun is partially hidden, forming a partial (or, possibly, an annular) eclipse.
2. n. The gray fringing border which surrounds the dark umbra or nucleus of a sun-spot.
3. n. In painting, the boundary of shade and light, where the one blends with the other, the gradation being almost imperceptible.
4. n. A surrounding fringe or border, as in the distribution of metamorphic effects produced by an igneous mass.

Wiktionary

1. n. A partially shaded area around the edges of a shadow, especially an eclipse.
2. n. astronomy A region around the edge of a sunspot, darker than the sun's surface but lighter than the middle of the sunspot.
3. n. figuratively An area of uncertainly or intermediacy between two mutually exclusive states or categories.
4. n. figuratively An area that lies on the edge of something; a fringe.
5. n. Something related to, connected to, and implied by, the existence of something else that is necessary for the second thing to be full and complete in its essential aspects.
6. n. medicine (in "ischaemic penumbra", after a stroke) A region of the brain that has lost only some of its blood supply, and retains structural integrity but has lost function.

GNU Webster's 1913

1. n. An incomplete or partial shadow.
2. n. (Astron.) The shadow cast, in an eclipse, where the light is partly, but not wholly, cut off by the intervening body; the space of partial illumination between the umbra, or perfect shadow, on all sides, and the full light.
3. n. (Paint.) The part of a picture where the shade imperceptibly blends with the light.

WordNet 3.0

1. n. a fringe region of partial shadow around an umbra