Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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