brain

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The portion of the vertebrate 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.
• noun A functionally similar portion of the invertebrate nervous system.
• noun Intellectual ability; intellect.
• noun Exceptional intellectual ability; intelligence.
• noun Informal A highly intelligent person.
• noun The primary director or planner, as of an organization or movement.
• noun The control center, as of a ship, aircraft, or spacecraft.
• transitive verb To hit on the head or kill by hitting on the head.
• idiom Informal (beat (one's) brains (out)) To exert or expend great mental effort.
• idiom (on the brain) Obsessively in mind.
• idiom (brain/brains) To explore another's ideas through questioning.
• idiom (rack (one's) brain) To think long and hard.

from The Century Dictionary.

• To dash out the brains of; kill by beating in the skull.
• Figuratively, to destroy; defeat; balk; thwart.
• To get into the brain; conceive; understand.
• noun In anatomy, the soft grayish and whitish mass filling the cranial cavity of a vertebrate, consisting of ganglionic nerve-cells and nerve-fibers, with the requisite sustentacular and vascular tissue; the encephalon (which see); the part of the cerebrospinal axis which is contained in the cranium.
• noun In entomology, the principal ganglion of the nervous system, situated in the head, over the esophagus, and formed by the coalescence of serveral supra-esophageal ganglia.
• noun The same or a corresponding portion of the nervous system in many other invertebrates.
• noun Understanding; intellectual power; fancy; imagination: commonly in the plural: as, a man of brains; “my brain is too dull,”

from the GNU version of the Collaborative International Dictionary of English.

• noun (Anat.) The whitish mass of soft matter (the center of the nervous system, and the seat of consciousness and volition) which is inclosed in the cartilaginous or bony cranium of vertebrate animals. It is simply the anterior termination of the spinal cord, and is developed from three embryonic vesicles, whose cavities are connected with the central canal of the cord; the cavities of the vesicles become the central cavities, or ventricles, and the walls thicken unequally and become the three segments, the fore-, mid-, and hind-brain.
• noun (Zoöl.) The anterior or cephalic ganglion in insects and other invertebrates.
• noun The organ or seat of intellect; hence, the understanding.
• noun rare The affections; fancy; imagination.
• noun informal a very intelligent person.
• noun informal the controlling electronic mechanism for a robot, guided missile, computer, or other device exhibiting some degree of self-regulation.
• noun [Low] to have constantly in one's thoughts, as a sort of monomania.
• noun a decision requiring little or no thought; an obvious choice.
• noun the bony or cartilaginous case inclosing the brain.
• noun (Zoöl) a massive reef-building coral having the surface covered by ridges separated by furrows so as to resemble somewhat the surface of the brain, esp. such corals of the genera Mæandrina and Diploria.
• noun (Med.) brain weariness. See Cerebropathy.
• noun (Med.) fever in which the brain is specially affected; any acute cerebral affection attended by fever.
• noun calcareous matter found in the pineal gland.
• transitive verb To dash out the brains of; to kill by beating out the brains.
• transitive verb obsolete To conceive; to understand.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The control center of the central nervous system of an animal located in the skull which is responsible for perception, cognition, attention, memory, emotion, and action.
• noun informal An intelligent person.
• noun UK, plurale tantum A person who provides the intelligence required for something.
• noun in the plural Intellect.
• noun By analogy with a human brain, the part of a machine or computer that performs calculations.
• verb transitive To dash out the brains of; to kill by smashing the skull.
• verb transitive, slang To strike (someone) on the head.
• verb transitive, figuratively To destroy; to put an end to.
• verb transitive To conceive in the mind; to understand.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun that which is responsible for one's thoughts and feelings; the seat of the faculty of reason
• verb kill by smashing someone's skull
• noun that part of the central nervous system that includes all the higher nervous centers; enclosed within the skull; continuous with the spinal cord