excursus

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.

   “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.”

   “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.”

   “The authors devote a fascinating excursus to the Western imagination’s preoccupation with serpents—from Eden to esoterica—and how this primed the ground for Kuṇḍalinī’s reception.”