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penumbra (n.)

1660s, "partially shaded region around the shadow of an opaque body, a partial shadow," from Modern Latin penumbra "partial shadow outside the complete shadow of an eclipse," coined 1604 by Kepler from Latin pæne "nearly, almost, practically," which is of uncertain origin, + umbra "shadow" (see umbrage). Figurative use is by 1801. Related: Penumbral.

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. [Century Dictionary]

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Definitions of penumbra from WordNet

penumbra (n.)
a fringe region of partial shadow around an umbra;
From wordnet.princeton.edu