How would one describe a concept that is "nebulous"?

The term "nebulous" refers to something that is not clear, defined, or concrete; it suggests a lack of clarity or precision. Therefore, describing a concept as nebulous means that it is vague and unclear. This aligns perfectly with the choice that identifies such characteristics, reinforcing the idea that the definition encapsulates the essence of the term.

In a broader context, concepts that are considered nebulous often lack the specificity needed for effective communication or understanding, making them open to varied interpretations. This highlights the importance of clarity, especially in discussions or arguments, where a nebulous concept can lead to confusion or miscommunication. The other choices contrast sharply with this definition; for example, being "clear and well-defined" directly contradicts the notion of being nebulous, while "fundamentally different" and "fearless and adventurous" do not capture the essence of ambiguity that the adjective implies.