excursus

1. A lengthy, appended exposition of a topic or point.
2. A digression.

[Latin, from past participle of excurrere, to run out. See excursion.]

noun

An instance of digressing: aside, deviation, digression, divagation, divergence, divergency, excursion, irrelevancy, parenthesis, tangent. See approach/retreat.

excursus (plural ‐suses), a digression in which some point is discussed at length; or an appendix devoted to detailed examination of some topic held over from the main body of the text.

adjective: excursive.

The noun has one meaning:

Meaning #1: a message that departs from the main subject
  Synonyms: digression, aside, divagation, parenthesis