When an element exists in two or more forms in the same physical state, it exhibits allotropy or allotropism. 

Allotropy refers to the property of some chemical elements to exist in two or more different forms, known as allotropes, in the same physical state (solid, liquid, or gas). 

Allotropes are different forms of the same element that have different physical properties but the same chemical properties. 

Examples of elements that exhibit allotropy include carbon (graphite, diamond), sulphur etc