Which term describes someone or something that is hard to grasp or understand?

The term "elusive" refers to something that is difficult to catch, understand, or define. It conveys the idea of being evasive or hard to pin down, whether that refers to a concept, an idea, or even a person. In the context of the question, it aptly describes something that is challenging to comprehend or grasp fully. For instance, abstract concepts or intricate philosophies may be referred to as elusive due to their complexity and the difficulty they present in terms of understanding.

The other terms offer contrasting meanings: "accessible" suggests something that is easy to reach or understand; "transparent" implies clarity and openness, often used to describe something easily observable or comprehensible; while "obvious" indicates that something is clear and evident. Thus, "elusive" stands out as the most fitting option since it captures the essence of being hard to grasp or understand.