An explanation of a word or passage in a book is known as what?

The correct term for an explanation of a word or passage in a book is annotation. This involves adding notes or comments that clarify the text, provide interpretations, or offer additional information. Annotations can enhance understanding by pinpointing key ideas, defining complex terms, or relating concepts to wider contexts.

In contrast, a glossary specifically provides definitions of key terms found within a text, usually located at the end of a book or chapter. Footnotes serve a similar function by offering explanations or references at the bottom of the page where the relevant text appears. A summary distills the main points or themes of a larger work into a brief overview, which is different from providing direct explanations of individual words or passages. Each of these terms plays a unique role in understanding and engaging with a text, but it is annotations that directly explain specific content.