n 1  :  the action of intersecting or crossing (as of nerve fibers) esp. in the form of an X - see DECUSSATION OF PYRAMIDS   2 a  :  a band of nerve fibers that connects unlike centers on opposite sides of the nervous system  b  :  a crossed tract of nerve fibers passing between centers on opposite sides of the central nervous system  :  COMMISSURE .
 
 
 
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