1. The property of certain chemical substances occurring in different crystalline forms: One example is calcium carbonate that exists as calcite and as aragonite.

Allomorphism refers to the property possessed by certain substances of assuming a different crystalline form while remaining unchanged in chemical constitution.
2. A variant phonological representation of a morpheme: Examples of allomorphism are the final sounds of "bets" and "beds", as well as, "horses" and "oxen" which are allomorphs of the English plural morpheme.