The Collaborative International Dictionary of English v.0.48:

Decussation \De`cus*sa"tion\, n. [L. decussatio.]
   Act of crossing at an acute angle, or state of being thus
   crossed; an intersection in the form of an X; as, the
   decussation of lines, nerves, etc.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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