penumbra

penumbra is defined in Webster's Unabridged Dictionary (1913) with 2 senses. The full text of each entry is reproduced verbatim below.

Definitions

1.(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. Sir I. Newton. Note: The faint shade surrounding the dark central portion of a solar spot is also called the penumbra, and sometimes umbra.
2.(Paint.) The part of a picture where the shade imperceptibly blends with the light.

Source: Webster's Unabridged Dictionary, 1913 edition (public domain, via GCIDE / Project Gutenberg).

Definition-first questions whose answer is penumbra.

Definitions: Webster's Unabridged Dictionary, 1913 edition (public domain, via GCIDE / Project Gutenberg).
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