brain \brain\ br=an, n. oe. brain, brein, as. bragen,
   braegen; akin to lg. braumgen, bregen, d. brein, and
   perh. to gr. bre`gma, brechmo`s, the upper part of head, if
   beta = phi. root95.
   1913 webster
   1. 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.
      1913 webster

   note: in the brain of man the cerebral lobes, or largest part
         of the forebrain, are enormously developed so as to
         overhang the cerebellum, the great lobe of the
         hindbrain, and completely cover the lobes of the
         midbrain. the surface of the cerebrum is divided into
         irregular ridges, or convolutions, separated by grooves
         the so-called fissures and sulci, and the two
         hemispheres are connected at the bottom of the
         longitudinal fissure by a great transverse band of
         nervous matter, the corpus callosum, while the two
         halves of the cerebellum are connected on the under
         side of the brain by the bridge, or pons varolii.
         1913 webster

   2. zool. the anterior or cephalic ganglion in insects and
      other invertebrates.
      1913 webster

   3. the organ or seat of intellect; hence, the understanding;
      as, use your brains. " my brain is too dull." --sir w.
      scott.
      1913 webster

   note: in this sense, often used in the plural.
         1913 webster

   4. the affections; fancy; imagination. r. --shak.
      1913 webster

   5. a very intelligent person. informal
      pjc

   6. the controlling electronic mechanism for a robot, guided
      missile, computer, or other device exhibiting some degree
      of self-regulation. informal
      pjc

   to have on the brain, to have constantly in one's thoughts,
      as a sort of monomania. low

   no-brainer a decision requiring little or no thought; an
      obvious choice. slang
      1913 webster

   brain box or brain case, the bony or cartilaginous case
      inclosing the brain.

   brain coral, brain stone coral zool., 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
      maeandrina and diploria.

   brain fag med., brain weariness. see cerebropathy.

   brain fever med., fever in which the brain is specially
      affected; any acute cerebral affection attended by fever.
      

   brain sand, calcareous matter found in the pineal gland.
      1913 webster
see also:
to have on the brain no-brainer brain box brain case brain coral brain stone coral 
maeandrina diploria brain fag cerebropathy brain fever 
brain sand 

no-brainer
     n : anything that requires little thought