Crystal polymorphism is the existence of different crystal structures for the same chemical substance. Each crystal polymorph can have its own mass density, melting behavior and range of stability on a phase diagram. A different external shape of the same crystal structure alone does not establish a different crystal polymorph.[ref]
polymorph. IUPAC Compendium of Chemical Terminology, fifth edition, entry “polymorph” (15225), definition and Note 2
At fixed temperature and pressure, compare the chemical potentials of the possible crystal polymorphs. The lower value determines the stable bulk thermodynamic phase; equality permits phase coexistence when neither form is beaten by another thermodynamic phase. A higher-chemical potential form can persist through metastability, so finding it in a sample does not by itself locate an equilibrium boundary.
A solid thermodynamic phase with a particular crystal structure, among distinct crystalline forms of the same chemical substance. Crystal polymorphism names the existence of these alternatives; a crystal polymorph names one of the forms.
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