In general, polymorphism describes multiple possible states for a single property.
In materials science polymorphism is the ability of a solid material to exist in more than one form or crystal structure. Diamond and graphite are examples of polymorphs of carbon. α-ferrite, austenite, and δ-ferrite are polymorphs of iron. When found in elemental solids the condition is also called Allotropy.
In object-orientated programming languages, polymorphism describes methods of different objects that have the same name (and usually the same parameter list), so that calling a.add(1) invokes a different function than b.add(1).