Source : Webster's Revised Unabridged Dictionary (1913)

Chiasm \Chi"asm\, Chiasma \Chi*as"ma\, n. [NL. chiasma, fr. Gr.
   ? two lines placed crosswise, fr. ? to mark with a [chi].]
   (Anat.)
   A commissure; especially, the optic commissure, or crucial
   union of the optic nerves. -- {Chi*as"mal}, a..

Source : WordNet®

chiasm
     n : an intersection or crossing of two tracts in the form of the
         letter X [syn: {chiasma}, {decussation}]