What term refers to a person or thing that is mysterious or difficult to understand?

The term that best describes a person or thing that is mysterious or difficult to understand is "enigma." This word is often used to signify something that baffles or perplexes individuals, encapsulating the essence of a mystery or a situation that lacks clarity.

In contrast, options like “puzzle” imply a specific problem or challenge to solve, which while similar, does not necessarily convey the depth of mystery associated with an “enigma.” The terms “clarity” and “solution” indicate understanding or resolution, which directly oppose the notion of something being hard to grasp or mysterious. Therefore, "enigma" is the most fitting choice, as it captures the complex and elusive nature of the subject in question.