The Collaborative International Dictionary of English v.0.48:

Dialysis \Di*al"y*sis\, n.; pl. Dialyses. [L., separation, fr.
   Gr. ?, fr. ? to part asunder, dissolve; dia` through + ? to
   loose.]
   1. (Gram.) Di[ae]resis. See Di[ae]resis, 1.
      [1913 Webster]

   2. (Rhet.) Same as Asyndeton.
      [1913 Webster]

   3. (Med.)
      (a) Debility.
      (b) A solution of continuity; division; separation of
          parts.
          [1913 Webster]

   4. (Chem.) The separation of different substances in
      solution, as crystalloids and colloids, by means of their
      unequal diffusion, especially through natural or
      artificial membranes.
      [1913 Webster]

WordNet (r) 3.0 (2006):

dialysis
    n 1: separation of substances in solution by means of their
         unequal diffusion through semipermeable membranes