Decussation describes the crossing over of a tract to the side of the nervous system opposite to where the axons originated.
This happens in various spinal tracts from body to brain or vice versa, such that we "feel" (sensory) the left side of our body by sensing touch with the right side of our brain, and likewise we "move" (motor) the right side of our body by impulses generated by the left side of our brain.
The term decussation derives from Latin "decussatus", of the word "decussare" which translates into ‘divide crosswise.’
This also relates to the crossing over like that of the figure X, for which the word decussis means. Interesting the Roman numeral for the number 10, which is written "X," derives from this root word into "decem, " for the English number ‘ten.’