Ability of a solid material to exist in more than one form or crystal structure
In crystallography, polymorphism is the phenomenon where a compound or element can crystallize into more than one crystal structure.
The preceding definition has evolved over many years and is still under discussion today.[1][2][3] Discussion of the defining characteristics of polymorphism involves distinguishing among types of transitions and structural changes occurring in polymorphism versus those in other phenomena.
^Bernstein, Joel (2002). Polymorphism in Molecular Crystals. New York, USA: Oxford University Press. pp. 1–27. ISBN0198506058.