noun, plural umbras, umbrae [uhm-bree] .
shade; shadow.
the invariable or characteristic accompaniment or companion of a person or thing.
the complete or perfect shadow of an opaque body, as a planet, where the direct light from the source of illumination is completely cut off. Compare penumbra ( def 1a ).
the dark central portion of a sunspot. Compare penumbra ( def 1b ).
a phantom or shadowy apparition, as of someone or something not physically present; ghost; spectral image.

1590–1600; < Latin: shade, shadow

umbral, adjective Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
umbra (ˈʌmbrə)
n , pl -brae, -bras
1.  a region of complete shadow resulting from the total obstruction of light by an opaque object, esp the shadow cast by the moon onto the earth during a solar eclipse
2.  the darker inner region of a sunspot
[C16: from Latin: shade, shadow]

Collins English Dictionary - Complete & Unabridged 10th Edition
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Word Origin & History

1599, "phantom, ghost," a fig. use from L. umbra "shade, shadow" (see umbrage). Astronomical sense of "shadow cast by the earth or moon during an eclipse" is first recorded 1679. Meaning "an uninvited guest accompanying an invited one" is from 1696.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Science Dictionary
umbra   (ŭm'brə)  Pronunciation Key 
Plural umbras or umbrae (ŭm'brē)
  1. 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.

  2. The dark central region of a sunspot. Compare penumbra.

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