The term 'caption in a diet ad' can be summarized as a phrase or sentence that is used to accompany an advertisement for a diet product. This phrase or sentence typically serves to explain the benefits of the product or to encourage the reader to purchase it. The crossword solution for this term is 'PREAMBLE, ' which means a short introductory statement or explanation. Preambles are often used in advertisements to provide context and to explain why the product is beneficial.