The Collaborative International Dictionary of English v.0.48:

Commissure \Com*mis"sure\ (? or ?; 134-6), n. [L. commissura a
   joining together: cf. F. commissure. See Commit.]
   1. A joint, seam, or closure; the place where two bodies, or
      parts of a body, meet and unite; an interstice, cleft, or
      juncture.
      [1913 Webster]

   2. (Anat. & Zool.)
      (a) The point of union between two parts, as the angles of
          the lips or eyelids, the mandibles of a bird, etc.
      (b) A collection of fibers connecting parts of the brain
          or spinal marrow; a chiasma.
          [1913 Webster]

   3. (Bot.) The line of junction or cohering face of two
      carpels, as in the parsnip, caraway, etc.
      [1913 Webster]

WordNet (r) 3.0 (2006):

commissure
    n 1: a bundle of nerve fibers passing from one side to the other
         of the brain or spinal cord