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Dimorphism is the property of some minerals to exist in two different crystal systems, while still having the same chemical formula. An example: CaCO3, which exists as trigonal calcite or orthorhombic aragonite. A mineral's dimorph is the mineral that it is dimorphic with. Dimorphism is not to be confused with allotropy, which refers to a difference in crystal structure of pure elements.
Trimorphism & Polymorphism
Trimorphism is the term used when a mineral has a total of three dimorphs, while the term Polymorphism is used when there are four or more. Silica, SiO2, is a good example, being found in nature as quartz, tridymite, cristobalite, coesite, and stishovite. Pleomorphism is a synonym of polymorphism.
References
Dimorphism (geology) Wikipedia(Text) CC BY-SA