Polymorphism
The ability of a single substance to exist in more than one crystalline form.
Unit
Dimensionless
In depth
Polymorphs differ in lattice arrangement and therefore in density, hardness, solubility and melting point, though their chemical composition is identical. Polymorphism is critically important in pharmaceuticals, where different forms of the same drug can dissolve at very different rates.
Examples in the real world
Calcium carbonate as calcite and aragonite; titanium dioxide as rutile and anatase; the many phases of ice.