Which word describes something that is fundamentally different from something else?

The term that best describes something fundamentally different from something else is "disparate." This word conveys a sense of distinctness and highlights the lack of similarity between two subjects, indicating that they come from different sources or have different characteristics. When things are described as disparate, it emphasizes their varied and contrasting nature.

In contrast, "identical" suggests complete sameness, implying that there are no differences at all between the items in question. "Similar" indicates that there are some commonalities between the items, but it does not denote any fundamental difference; rather, it emphasizes resemblance. "Parallel" refers to aspects that run alongside one another, potentially in similarity but also can indicate sameness in direction or purpose, which does not align with the concept of fundamental difference. Thus, "disparate" is the most fitting word in this context.