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Definition of umbra - 4 dictionary results
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um⋅bra

  [uhm-bruh]
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–noun, plural -bras, -brae  [-bree] .
1. shade; shadow.
2. the invariable or characteristic accompaniment or companion of a person or thing.
3. Astronomy.
a. 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).
b. the dark central portion of a sunspot. Compare penumbra (def. 1b).
4. a phantom or shadowy apparition, as of someone or something not physically present; ghost; spectral image.
Origin:
1590–1600; < L: shade, shadow

Related forms:
umbral, adjective
Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2009.
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um·bra   (ŭm'brə)   
n.   pl. um·bras or um·brae (-brē)

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

  2. 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.


[Latin, shadow.]
um'bral adj.
The American Heritage® Dictionary of the English Language, Fourth Edition
Copyright © 2009 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
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Word Origin & History

umbra 
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, © 2001 Douglas Harper
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Encyclopedia

umbra

that part of a shadow in which all light from a given source is excluded. The shadow from a point source of illumination is essentially all umbra, but that from a source of some size (as from the Sun) consists of a small umbra and a much larger partial shadow called the penumbra. Thus, in an eclipse of the Sun, the regions within the umbra experience a total eclipse and those within the penumbra, partial eclipse. The term is also used for the dark central portion of a sunspot.

Learn more about umbra with a free trial on Britannica.com.

Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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