The Collaborative International Dictionary of English v.0.48:

Chiasm \Chi"asm\ (k[imac]"[a^]z'm), Chiasma \Chi*as"ma\
   (k[-i]*[a^]z"m[.a]), n. [NL. chiasma, fr. Gr. chi`asma two
   lines placed crosswise, fr. ? to mark with a [chi].] (Anat.)
   A commissure; an intersection or crossing of two tracts;
   especially, the optic commissure, or crucial union of the
   optic nerves. -- Chi*as"mal, a..
   [1913 Webster] chiasmatic
   chiasmal

WordNet (r) 3.0 (2006):

chiasma
    n 1: an intersection or crossing of two tracts in the form of
         the letter X [syn: chiasma, chiasm, decussation]