not transparent or translucent; impenetrable to light; not allowing light to pass through.
not transmitting radiation, sound, heat, etc.
not shining or bright; dark; dull.
hard to understand; not clear or lucid; obscure: The problem remains opaque despite explanations.
dull, stupid, or unintelligent.
something that is opaque.
Photography. a coloring matter, usually black or red, used to render part of a negative opaque.
verb (used with object), opaqued, opaquing.
Photography. to cover up blemishes on (a negative), especially for making a printing plate.
to cause to become opaque.

1375–1425; late Middle English opake < Latin opācus shaded

opaquely, adverb
opaqueness, noun
subopaque, adjective
subopaquely, adverb
subopaqueness, noun

1. murky, cloudy, muddy.
Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
opaque (əʊˈpeɪk)
1.  not transmitting light; not transparent or translucent
2.  not reflecting light; lacking lustre or shine; dull
3.  not transmitting radiant energy, such as electromagnetic or corpuscular radiation, or sound
4.  hard to understand; unintelligible
5.  unintelligent; dense
6.  photog an opaque pigment used to block out particular areas on a negative
vb , opaques, opaquing, opaqued
7.  to make opaque
8.  photog to block out particular areas, such as blemishes, on (a negative), using an opaque
[C15: from Latin opācus shady]

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

c.1420, opake, from L. opacus "shaded, shady, dark," of unknown origin. Spelling infl. after c.1650 by Fr. opaque (c.1500), from the L.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

opaque o·paque (ō-pāk')
Impenetrable by light; neither transparent nor translucent.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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American Heritage
Science Dictionary
opaque  [%PREMIUM_LINK%]     (ō-pāk')  Pronunciation Key 
Resistant to the transmission of certain kinds of radiation, usually light. Metals and many minerals are opaque to light, while being transparent to radio waves and neutrinos. Compare translucent, transparent.

The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
Some of the imagery in the exhibition is comprehensible to informed outsiders,
  while some is ambiguous or completely opaque.
Pretending to be a medical expert is possible precisely because medical
  knowledge is so specialized and opaque.
The fabric is opaque translucent so the taillights shine through, and small
  motors pull the fabric back to reveal the headlights.
Lost is so opaque that you need networked smarts to fully enjoy it.
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