Which term refers to different forms of the same element?

The term that refers to different forms of the same element is allotrope. Allotropes are distinct physical forms in which an element can exist, and they have different structural configurations and properties. For instance, carbon has several allotropes, including graphite, diamond, and fullerenes. Each of these forms exhibits unique characteristics despite being made up solely of carbon atoms.

Considering the other terms, isotopes refer specifically to variations of elements that have the same number of protons but different numbers of neutrons, leading to different atomic masses. Homologues typically pertain to compounds in chemistry that have a similar structure or functional group, often in a series. Polymers are large molecules made up of repeating units (monomers), and they are not directly related to different forms of a single element. Thus, allotrope is the appropriate term that captures the concept of different structural forms of the same element.