Noun

1. A fuller treatment (in a separate section) of a particular part of the text of a book, especially a classic.
2. A narrative digression, especially to discuss a particular issue.
   • 1979, Kyril Bonfiglioli, After You with the Pistol, Penguin 2001, p. 204:

     Here is what us scholars call an excursus. If you are an honest man the following page or two can be of no possible interest to you.

   • 2007, Glen Bowersock, ‘Provocateur’, London Review of Books 29:4, p. 16:

     In his excursus on the Jewish people at the opening of the fifth book of his Histories [...], Tacitus was at a loss to uncover any deep cause for the war that broke out in 66.