gray matter

Anatomy. nerve tissue, especially of the brain and spinal cord, that contains fibers and nerve cell bodies and is dark reddish-gray. Compare white matter.
Informal. brains or intellect.

1830–40 Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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American Heritage
Medical Dictionary

gray matter n.
Brownish-gray nerve tissue, especially of the brain and spinal cord, composed of nerve cell bodies and their dendrites and some supportive tissue. Also called gray substance, substantia grisea.

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
gray matter  
The brownish-gray tissue of the vertebrate brain and spinal cord, made up chiefly of the cell bodies and dendrites of neurons. Compare white matter.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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American Heritage
Idioms & Phrases

gray matter

Brains, intellect, as in If you'd only use your gray matter, you'd see the answer in a minute. This expression refers to actual brain tissue that is gray in color. Agatha Christie's fictional detective, Hercule Poirot, constantly alludes to using the little gray cells for solving a crime. [Late 1800s]

The American Heritage® Dictionary of Idioms by Christine Ammer.
Copyright © 1997. Published by Houghton Mifflin.
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Example sentences
The brain has not yet revealed its mysterious mechanism of gray matter.
The technique revealed never-before-seen patterns in the white matter and gray
  matter of the human brain.
After a week, the jugglers were already developing extra gray matter in some
  brain areas.
Now comes a study finding that it's possible to increase the brain's gray
  matter quite quickly-in only a matter of days.
Images for gray matter
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