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]

1550s, "faint sketch, imperfect representation," from Latin adumbrationem (nominative adumbratio) "a sketch in shadow, sketch, outline," noun of action from past-participle stem of adumbrare "to cast a shadow, overshadow," in painting, "represent (a thing) in outline," from ad- "to" (see ad-) + umbrare "to cast in shadow," from PIE root *andho- "blind; dark" (see umbrage).

"ancestor-worship, deification of the shades of the dead," 1886 (Huxley); from Latinized combining form of Greek skia "shade, shadow" (see Ascians); also see theism.

1760 "gloomy, shadowy" (earlier sombrous, c. 1730), from French sombre "dark, gloomy," from Old French sombre (14c.), from an adjective from Late Latin subumbrare "to shadow," from sub "under" (see sub-) + umbra "shade, shadow" (perhaps from a suffixed form of PIE *andho- "blind, dark;" see umbrage). Related: Somberly; somberness.

1620s, "sham-fight for exercise or practice," from Latinized form of Greek skiamakhia "shadow-fighting, a sham fight," from skia "shade, shadow" (see Ascians) + makhē "battle" (see -machy). The notion in the Greek word is said sometimes to be "fighting in the shade" (i.e. practicing in school; ancient teachers taught in shaded public places such as porches and groves), but it seems also to have had a sense of "fighting with shadows, shadow-boxing." In English, often figurative, of futile combat with an imaginary enemy.

1590s, "of or pertaining to the Periscii," the inhabitants of the Polar circle in ancient Greek imagination, literally (those) "throwing a shadow all round," from peri "round about" (see peri-) + skia "shade, shadow" (see, and compare, Ascians). So called because their shadows would revolve around them during the course of a summer day, when the sun is always above the horizon there.