umbra

A dark area, especially the blackest part of a shadow from which all light is cut off. See Synonyms at shade.

Astronomy

1. The completely dark portion of the shadow cast by the earth, moon, or other body during an eclipse.

2. The darkest region of a sunspot.

The darkest part of a shadow, especially the cone-shaped region of full shadow cast by Earth, the Moon, or another body during an eclipse. In a full lunar eclipse, which generally lasts for one or two hours, the entire disk of the Moon is darkened as it passes through the umbra. During this period the Moon takes on a faint reddish glow due to illumination by a small amount of sunlight that is refracted through the Earth's atmosphere and bent toward the darkened Moon; the reddish tint is caused by the filtering out of blue wavelengths as the sunlight passes through the Earth's atmosphere, leaving only the longer wavelengths on the red end of the spectrum. See Note at eclipse.

The dark central region of a sunspot. Compare penumbra.