Which word best describes a title or chapter heading?

The word "caption" is the best choice to describe a title or chapter heading because it refers specifically to a brief explanation or title that accompanies an illustration, photograph, or section of a text. In the context of written works, a caption succinctly summarizes the content of the corresponding section or provides necessary context, much like how a chapter heading introduces the themes and main ideas of that part of a book.

On the other hand, "clad" pertains to being covered or dressed, which does not relate to text or headings. "Cache" refers to a storage space or a collection of items hidden away, which is also unrelated to the topic of titles or headings. "Clarify" means to make something clearer or easier to understand, but it does not denote a title or heading itself. Hence, "caption" aligns perfectly with the function and purpose of a title or chapter heading.