The Collaborative International Dictionary of English v.0.48:

Chorisis \Cho"ri*sis\, n. [NL., fr. Gr. ? separation.] (Bot.)
   The separation of a leaf or floral organ into two more parts.
   [1913 Webster]

   Note: In collateral chorisis the parts are side by side. --
         In parallel or median chorisis they are one in front of
         another.
         [1913 Webster]