What term describes different forms of the same element, such as carbon in diamond and graphite?

The term that describes different forms of the same element is "allotrope." Allotropes are variations of an element that have distinct physical and chemical properties due to differences in the arrangement of atoms. For example, carbon can exist in several allotropes, including diamond, where the carbon atoms are arranged in a three-dimensional lattice structure, and graphite, where the carbons are arranged in layers that can slide over each other. This structural distinction leads to vastly different properties—diamond is extremely hard and transparent, whereas graphite is soft and opaque.

In contrast, the other terms related to the question do not encompass the concept of different forms of a single element in this way. Isotopes refer to variations of an element that differ in the number of neutrons, leading to different atomic masses but not different structures or forms. Molecules are combinations of two or more atoms, which can be the same or different elements, while compounds specifically refer to substances formed from two or more different elements chemically bonded together. Thus, "allotrope" is the precise term to describe the different forms of carbon mentioned in the question.