Which term would best describe speech that is unclear and often difficult to understand?

The term that best describes speech that is unclear and often difficult to understand is "barylalia." This term specifically refers to a speech disorder characterized by unclear or slurred speech, often making it difficult for listeners to comprehend what is being said. Barylalia highlights problems with articulation and clarity in speech, which aligns perfectly with the definition in the question.

In contrast, "articulate" indicates clear and effective speech, emphasizing the ability to express thoughts coherently. "Coherent" refers to logical and consistent expression, where ideas are presented in an understandable manner. "Fluent" denotes a smooth, effortless manner of speaking, usually implying that the speaker is articulate and coherent as well. Thus, these terms do not convey the notion of unclear or difficult speech, which is why they are not suitable choices for this question.