Anatomy, Zoology. the part of the central nervous system enclosed in the cranium of humans and other vertebrates, consisting of a soft, convoluted mass of gray and white matter and serving to control and coordinate the mental and physical actions.
Zoology. (in many invertebrates) a part of the nervous system more or less corresponding to the brain of vertebrates.
Sometimes, brains. (used with a plural verb) understanding; intellectual power; intelligence.
the brain as the center of thought, understanding, etc.; mind; intellect.
brains, Slang. a member of a group who is regarded as its intellectual leader or planner: The junior partner is the brains of the firm.
Informal. a very intelligent or brilliant person.
the controlling or guiding mechanism in a computer, robot, pacemaker, etc.
the part of a computer system for coordination or guidance, as of a missile.
verb (used with object)
to smash the skull of.
Slang. to hit or bang (someone) on the head.
beat one's brains out, Informal. to try very hard to understand and work out a problem, remember something, etc.: She beat her brains out studying for the exam.
have something on the brain, to have an obsession; be occupied with: Lately I seem to have food on the brain.
pick someone's brains, to obtain information by questioning another person rather than by seeking it independently: He refused to prepare for the exam but counted on being able to pick his roommate's brains.

before 1000; Middle English; Old English bræg(e)n, bregen; cognate with Low German brägen, Dutch brein

brainlike, adjective
superbrain, noun

3. sense; capacity. See mind. Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2015.
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World English Dictionary
brain (breɪn)
1.  Technical name: encephalon the soft convoluted mass of nervous tissue within the skull of vertebrates that is the controlling and coordinating centre of the nervous system and the seat of thought, memory, and emotion. It includes the cerebrum, brainstem, and cerebellumRelated: cerebral, encephalic
2.  the main neural bundle or ganglion of certain invertebrates
3.  informal (often plural) intellectual ability: he's got brains
4.  informal shrewdness or cunning
5.  informal an intellectual or intelligent person
6.  informal (usually plural; functioning as singular) a person who plans and organizes an undertaking or is in overall control of an organization, etc
7.  an electronic device, such as a computer, that performs apparently similar functions to the human brain
8.  on the brain constantly in mind: I had that song on the brain
9.  pick someone's brain to obtain information or ideas from someone
10.  to smash the skull of
11.  slang to hit hard on the head
Related: cerebral, encephalic
[Old English brægen; related to Old Frisian brein, Middle Low German bregen, Greek brekhmos forehead]

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

O.E. brægen "brain," from P.Gmc. *bragnam (cf. M.L.G. bregen, O.Fris., Du. brein), from PIE base *mregh-m(n)o- "skull, brain" (cf. Gk. brekhmos "front part of the skull, top of the head"). The custom of using the plural to refer to the substance (literal or figurative), as opposed to the organ,
dates from 16c. Figurative sense of "intellectual power" is from late 14c.; meaning "a clever person" is first recorded 1914. The verb meaning "to dash the brains out" is from late 14c. Brain-dead is from 1976, popularized by the Karen Anne Quinlan case (brain death is from 1968); brain teaser is from 1923. Brain stem first recorded 1879, from German. Brain drain is attested from 1963. An O.E. word for "head" was brægnloca, which might be translated as "brain locker." In M.E., Brainsick (O.E. brægenseoc) meant "mad, addled."
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

brain (brān)
The portion of the central nervous system that is enclosed within the cranium, continuous with the spinal cord, and composed of gray matter and white matter. It is the primary center for the regulation and control of bodily activities, receiving and interpreting sensory impulses, and transmitting information to the muscles and body organs. It is also the seat of consciousness, thought, memory, and emotion. Also called encephalon.

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
brain  [%PREMIUM_LINK%]     (brān)  Pronunciation Key 

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  1. The part of the nervous system in vertebrates that is enclosed within the skull, is connected with the spinal cord, and is composed of gray matter and white matter. It is the control center of the central nervous system, receiving sensory impulses from the rest of the body and transmitting motor impulses for the regulation of voluntary movement. The brain also contains the centers of consciousness, thought, language, memory, and emotion. See more at brainstem, cerebellum, cerebrum.

  2. A bundle of nerves in many invertebrate animals that is similar to the vertebrate brain in function and position.

The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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American Heritage
Cultural Dictionary

brain definition

The central organ in the nervous system, protected by the skull. The brain consists of the medulla, which sends signals from the spinal cord to the rest of the brain and also controls the autonomic nervous system; the pons, a mass of nerve fibers connected to the medulla; the cerebellum, which controls balance and coordination; and the cerebrum, the outer layer of which, the cerebral cortex, is the location of memory, sight, speech, and other higher functions.

The cerebrum contains two hemispheres (the left hemisphere and the right hemisphere), each of which controls different functions. In general, the right hemisphere controls the left side of the body and such functions as spatial perception, whereas the left hemisphere controls the right side of the body and functions such as speech.

Under the cerebral cortex are the thalamus, the main relay center between the medulla and the cerebrum; and the hypothalamus, which controls blood pressure, body temperature, hunger, thirst, sex drive, and other visceral functions.

The American Heritage® New Dictionary of Cultural Literacy, Third Edition
Copyright © 2005 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
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American Heritage
Idioms & Phrases


In addition to the idioms beginning with brain, also see beat one's brains out; blow one's brains out; on one's mind (the brain); pick someone's brains; rack one's brains.

The American Heritage® Dictionary of Idioms by Christine Ammer.
Copyright © 1997. Published by Houghton Mifflin.
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Example sentences
New evidence shows that as the brain can change its mind, so can the mind change the brain.
And yet, for years, he suspected his mind had been taken over by parasites that had invaded his brain.
Pollen began looking for genes, near these deletions, that could affect brain
On the day before he was due to be sentenced to prison for his crimes, he had
  his brain scanned.
Image for brain
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