parenthesis vs excursus

parenthesis and excursus both are nouns.

Word	Noun	Adjective	Verb	Adverb
parenthesis	Yes	No	No	No
excursus	Yes	No	No	No

As nouns, parenthesis and excursus are synonyms defined as:

Other synonyms of parenthesis include aside, digression, divagation.

parenthesis (noun)	excursus (noun)
either of two punctuation marks (or) used to enclose textual material	a message that departs from the main subject
a message that departs from the main subject