The Collaborative International Dictionary of English v.0.48:

Metamorphism \Met`a*mor"phism\, n. (Geol.)
   The state or quality of being metamorphic; the process by
   which the material of rock masses has been more or less
   recrystallized by heat, pressure, etc., as in the change of
   sedimentary limestone to marble. --Murchison.
   [1913 Webster]

WordNet (r) 3.0 (2006):

metamorphism
    n 1: change in the structure of rock by natural agencies such as
         pressure or heat or introduction of new chemical substances