1) Aside 2) Content 3) Digression 4) Divagation 5) Literary concept 6) Message 7) Parenthesis 8) Subject matter 9) Substance 1) Digression 2) Dissertation 3) Divagation 
An excursus (from Latin excurrere, `to run out of`) is a short episode or anecdote in a work of literature. Often excursuses have nothing to do with the matter being discussed by the work, and are used to lighten the atmosphere in a tragic story, a similar function to that of satyr plays in Greek theatre. Sometimes they are used to provide backs.....
• (n.) A dissertation or digression appended to a work, and containing a more extended exposition of some important point or topic.
(1) A detailed analysis of a particular point or argument--epecially when added as an appendix at th
Ex·cur'sus noun [ Latin , from excurrere , excursum . See Excurrent .] A dissertation or digression appended to a work, and containing a more extended exposition of some important point or topic. 
An excursus is a moment where a text moves away from its main topic – it’s roughly similar to “digression.”
a detailed discussion of some point in a book, esp. one added as an appendix. · a digression or incidental excursion, as in a narrative.