excursus

[ek-skur-suh s] /ɛkˈskɜr səs/
noun, plural excursuses, excursus.
1.
a detailed discussion of some point in a book, especially one added as an appendix.
2.
a digression or incidental excursion, as in a narrative.
Origin
1795-1805; < Latin: a running out, sally, digression, derivative of excurrere to run out. See ex-1, course
British Dictionary definitions for excursus

excursus

/ɛkˈskɜːsəs/
noun (pl) -suses, -sus
1.
an incidental digression from the main topic under discussion or from the main story in a narrative
Word Origin
C19: from Latin: a running forth, from excurrere to run out