What is a section of a book that defines key terms called?

A glossary is a section of a book that specifically defines key terms, phrases, or concepts that are relevant to the material covered in the text. It typically appears at the end of the book and serves as a quick-reference guide for readers to understand specialized vocabulary or terminology used throughout the work. This feature is especially useful in academic and technical writing, where particular terms may not be commonly known to all readers.

The function of a glossary is to enhance comprehension and ensure that readers can engage with the content more effectively by providing clear definitions for potentially unfamiliar words. In contrast, other sections like an index organizes topics or terms based on their pages, a bibliography lists sources and references used in the text, and an appendix provides supplementary material that assists with the main text.