The term 'BIG REFERENCE BOOK' can be summarized as an ATLAS, which is a five-letter word. An ATLAS is a collection of maps and other information that is used as a reference book. It typically contains detailed maps of countries, cities, and other geographical features, as well as information about the history, culture, and economy of the area. An ATLAS is often used by students, researchers, and travelers to gain a better understanding of the world around them.