A more thorough explanation:

Definition: A sidenote is a note that is placed in the margin of a page, usually to provide additional information or clarification about the main text. It is also known as a marginal note.

Example: In a history book, there may be a sidenote that provides more information about a particular event or person mentioned in the main text. For instance, a sidenote may explain the significance of a battle or provide a brief biography of a historical figure.

Explanation: The example illustrates how a sidenote is used to provide additional information that is related to the main text. It is placed in the margin so that it does not disrupt the flow of the main text, but is still easily accessible to the reader. Sidenotes are commonly used in textbooks, academic papers, and other types of non-fiction writing.