The Collaborative International Dictionary of English v.0.48:

Metastasis \Me*tas"ta*sis\, n.; pl. Metastases. [L.,
   transition, fr. Gr. ?, fr. ? to place in another way; ? after
   + ? to place.]
   1. (Theol.) A spiritual change, as during baptism.
      [1913 Webster]

   2. (Med.) A change in the location of a disease, as from one
      part to another. --Dunglison.
      [1913 Webster]

   3. Especially: The migration of cancerous or malignant cells
      from the site of their origin to other parts of the body.
      Such cells may migrate to multiple new locations, making
      therapy more difficult and often leading to death.
      [PJC]

   4. (Physiol.) The act or process by which matter is taken up
      by cells or tissues and is transformed into other matter;
      in plants, the act or process by which are produced all of
      those chemical changes in the constituents of the plant
      which are not accompanied by a production of organic
      matter; metabolism.
      [1913 Webster]

WordNet (r) 3.0 (2006):

metastasis
    n 1: the spreading of a disease (especially cancer) to another
         part of the body

Moby Thesaurus II by Grady Ward, 1.0:

78 Moby Thesaurus words for "metastasis":
   anastrophe, avatar, catabolism, catalysis, chiasmus, communication,
   conduction, consubstantiation, contagion, convection, delivery,
   deportation, diapedesis, diffusion, displacement, dissemination,
   export, exportation, expulsion, extradition, heterotopia,
   hypallage, hyperbaton, hysteron proteron, import, importation,
   interchange, metabolism, metagenesis, metamorphism, metamorphosis,
   metathesis, metempsychosis, migration, mutant, mutated form,
   mutation, mutual transfer, osmosis, palindrome, parenthesis,
   passage, passing over, perfusion, permutation, reincarnation,
   sport, spread, spreading, synchysis, tmesis, transanimation,
   transduction, transfer, transfer of property, transference,
   transfiguration, transfigurement, transformation, transformism,
   transfusion, transit, transition, translation, translocation,
   transmigration, transmigration of souls, transmission, transmittal,
   transmittance, transmogrification, transmutation, transplacement,
   transplantation, transposal, transposition, transubstantiation,
   travel