1. The existence of a solid substance in different physical forms.

Tin, for example, has metallic and non-metallic crystalline forms. Carbon has two crystalline allotropes: diamond and graphite.

The term allotropes may also be used to refer to the molecular forms of an element; such as, a diatomic gas, even if there is only one such additional form.

2. The existence of an element in two or more distinct forms with different physical properties.