同素异形体，是指由同样的单一化学元素构成，但性质却不相同的单质。同素异形体之间的性质差异主要表现在物理性质上，化学性质上也有着活性的差异。

例如磷的两种同素异形体，红磷和白磷，它们的燃点分别是摄氏240 °C（464 °F）和40 °C（104 °F），充分燃烧之后的产物都是五氧化二磷；白磷（P₄）有剧毒，可溶于二硫化碳，红磷（P_n）无毒，却不溶于二硫化碳。同素异形体之间在一定条件下可以相互转化，这种转化是一种化学变化。

生活中最常见的，有碳的同素异形体：石墨、金刚石（即钻石）以及C₆₀（又称芙，或巴克球）；还有磷的同素异形体：白磷和红磷；另外，臭氧和氧气也是氧元素的两种同素异形体。

Allotropy or allotropism (fromGreekἄλλος (allos), meaning "other",andτρόπος (tropos), meaning "manner, form") is the property of some chemical elements to exist in two or more different forms, in the same physical state, known as allotropes of these elements. Allotropes are different structural modifications of an element; the atoms of the element are bonded together in a different manner. For example, the allotropes of carbon include diamond (where the carbon atoms are bonded together in a tetrahedral lattice arrangement), graphite (where the carbon atoms are bonded together in sheets of a hexagonal lattice), graphene (single sheets of graphite), and fullerenes (where the carbon atoms are bonded together in spherical, tubular, or ellipsoidal formations). The term allotropy is used for elements only, not for compounds. The more general term, used for any crystalline material, is polymorphism. Allotropy refers only to different forms of an element within the same phase (i.e. different solid, liquid or gas forms); these different states are not, themselves, considered to be examples of allotropy.