umbra
UM-brah
The darkest, central part of a shadow — where the light source is completely blocked. In an eclipse, the umbra is the cone of total shadow: if you stand within the Moon's umbra during a solar eclipse, the sun is entirely hidden and the sky goes dark. Surrounding the umbra is the penumbra, where the light source is only partially blocked. The umbra is the shadow's heart — the complete absence of direct light.
Etymology
Latin umbra, shadow, shade, ghost. The same root gives English umbrella (a little shadow), penumbra (almost shadow), and adumbrate (to foreshadow). The Romans heard ghosts in shadows.
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