excursus

[ ek-skur-suh s ]

/ ɛkˈskɜr səs /

a detailed discussion of some point in a book, especially one added as an appendix.

a digression or incidental excursion, as in a narrative.

Origin of excursus

1795–1805; < Latin: a running out, sally, digression, derivative of excurrere to run out. See ex-¹, course

The subject is complex, and calls for many an excursus and qualifying footnote; yet I think the main outlines are clear enough.

Winds Of Doctrine |George Santayana
That would involve an excursus on Irish topography, which would be, to say the least, out of place.

St. Bernard of Clairvaux's Life of St. Malachy of Armagh |H. J. Lawlor
Even the gentle Thackeray—if the excursus may be forgiven—is not wholly free from this failing.

Americanisms and Briticisms |Brander Matthews
This is not to say, of course, that the excursus in the second stage has been a loss and a defect.

Pagan & Christian Creeds |Edward Carpenter

It is needless to extend further the present excursus on the origin and history of modern pseudo-science.

Essays Upon Some Controverted Questions |Thomas H. Huxley

excursus

/ (ɛkˈskɜːsəs) /

an incidental digression from the main topic under discussion or from the main story in a narrative

C19: from Latin: a running forth, from excurrere to run out