What adjective describes concepts that are unclear or not defined?

The adjective that best describes concepts that are unclear or not defined is "nebulous." This term specifically conveys a sense of vagueness or ambiguity, often used to describe ideas or situations that lack clarity or definite boundaries. The origin of the word suggests something that is cloud-like or hazy, further emphasizing the notion of uncertainty.

While "ambiguous" also suggests a lack of clarity, it typically refers to situations where multiple interpretations are possible rather than a general vagueness. "Precise" implies clarity and exactness, which is the opposite of what is being described. "Evident" suggests something clear and obvious, further contrasting with the idea of unclear or undefined concepts. Therefore, "nebulous" is the most fitting choice for concepts that lack clear definition.