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Definition of penumbra - 6 dictionary results
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pe⋅num⋅bra

  [pi-nuhm-bruh]
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–noun, plural -brae  [-bree] , -bras.
1. Astronomy.
a. the partial or imperfect shadow outside the complete shadow of an opaque body, as a planet, where the light from the source of illumination is only partly cut off. Compare umbra (def. 3a).
b. the grayish marginal portion of a sunspot. Compare umbra (def. 3b).
2. a shadowy, indefinite, or marginal area.
Origin:
1660–70; < NL, equiv. to L paen- pen- + umbra shade

Related forms:
pe⋅num⋅bral, pe⋅num⋅brous, adjective
Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2009.
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pe·num·bra   (pĭ-nŭm'brə)   
n.   pl. pe·num·brae (-brē) or pe·num·bras

  1. A partial shadow, as in an eclipse, between regions of complete shadow and complete illumination. See Synonyms at shade.

  2. The grayish outer part of a sunspot.

  3. An area in which something exists to a lesser or uncertain degree: "The First Amendment has a penumbra where privacy is protected from governmental intrusion" (Joseph A. Califano, Jr.)

  4. An outlying surrounding region; a periphery: "Downtown Chicago and its penumbra also stand rejuvenated" (John McCormick).


[New Latin pēnumbra : Latin paene, almost + Latin umbra, shadow.]
pe·num'bral, pe·num'brous 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

penumbra 
1666, from Mod.L. penumbra "partial shadow outside the complete shadow of an eclipse," coined 1604 by Kepler from L. pæne "almost" + umbra "shadow."
Online Etymology Dictionary, © 2001 Douglas Harper
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Legal Dictionary

Main Entry: pe·num·bra
Pronunciation: pi-'n&m-br&
Function: noun
Inflected Form: plural -bras
1 : an area within which distinction or resolution is difficult or uncertain penumbra>
2 : an extension of protection, reach, application, or consideration; especially : a body of rights held to be guaranteed by implication from other rights explicitly enumerated in the U.S. Constitution penumbra where privacy is protected from governmental intrusion —Griswold v. Connecticut, 381 U.S. 479 (1965)> —see also Griswold v. Connecticut in the IMPORTANT CASES section —pe·num·bral /-br&l/ adjective
Merriam-Webster's Dictionary of Law, © 1996 Merriam-Webster, Inc.
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Medical Dictionary

Main Entry: pe·num·bra
Pronunciation: p&-'n&m-br&
Function: noun
Inflected Form: plural pe·num·brae /-(")brE, -"brI/ or pe·num·bras
: a blurred area in a radiograph at the edge of an anatomical structure —pe·num·bral /-br&l/ adjective
Merriam-Webster's Medical Dictionary, © 2002 Merriam-Webster, Inc.
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Encyclopedia

penumbra

(from Latin paene, "almost"; umbra, "shadow"), in astronomy, the outer part of a conical shadow, cast by a celestial body, where the light from the Sun is partially blocked-as compared to the umbra (q.v.), the shadow's darkest, central part, where the light is totally excluded. The definition of the term may be extended to include the partial shadow of any large source of light or electromagnetic radiation. An observer in the penumbra sees the Sun (or other light source) partially obscured. The term also is used for the outer portion of a sunspot.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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