The term CHAPTER OPENER refers to the first section of a book or CHAPTER which is usually used to introduce the reader to the topic and provide background information. The crossword solution for this term is SUB, which stands for subtitle. A subtitle is a phrase or sentence that follows the title of a book or CHAPTER and provides additional context about the topic. It is commonly used as a CHAPTER OPENER to give the reader an overview of what the book or CHAPTER will discuss.