1. a long digression or detailed discussion of a topic that is separate from the main subject of a book, essay, or speech
Usage: formal, academic; plural: excursuses or excursus
2. a supplementary section or appendix added to a text to explore a related subject in depth
Usage: formal, academic

Examples

• The historian included a lengthy excursus on medieval trade routes in the middle of her chapter on political alliances.
• The author's excursus on the etymology of the word distracted some readers from the main argument.
• Chapter 5 contains an excursus examining the philosophical foundations of the theory.
• Rather than interrupt the narrative flow, the author placed the excursus in an appendix.
• The professor's excursus into quantum mechanics was interesting but tangential to the biology lecture.
• The book's final excursus explores the cultural impact of the movement in the 21st century.