Polymorphism and Allotropy

It's the ability of a chemical substance to exist in more than one alternative crystal structure. It is related to allotropy and is described by crystal habit, amorphous, or crystal defect.

Polymorphism and Allotropy are same thing. Polymorphism is used for compounds and the allotropy is reserved for elements.

Allotropy or allotropism (from Greek ἄλλος (allos), meaning "other", and τρόπος (tropos), meaning "manner, form") is the property of some chemical elements to exist in two or more different forms, known as allotropes of these elements. Allotropes are different structural modifications of an element; the atoms of the element are bonded together in a different manner. For example, the allotropes of carbon include diamond (where the carbon atoms are bonded together in a tetrahedral lattice arrangement), graphite (where the carbon atoms are bonded together in sheets of a hexagonal lattice).