an act or an instance of leaving the main subject in an extended written or verbal expression of thought: the act or an instance of digressing in a discourse or other usually organized literary work https://www.merriam-webster.com/dictionary/digression

But, you haven't given enough context to be sure what would work best. Where do you want to use this term? Is someone describing what an author has done in a particular text - as in "He adds a side observation that..." or "He adds a parenthetical note to say that..."? Is the observation literally in parentheses? Is it in a footnote? In those cases, "author's note" or "parenthetical note" might do.

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Note added at 6 hrs (2019-01-15 22:41:52 GMT)
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Yes, but you still haven't said where he uses the word. Is it that directly in the text there comes a paragraph, whether in parentheses or not, that is labelled "экскурс"? Or, is your context separate from the book - a letter or press release or discussion in which the author says, "I have often inserted an 'экскурс' between chapters"?

Define excursus. excursus synonyms, excursus pronunciation, excursus translation, English dictionary definition of excursus. n. pl. ex·cur·sus·es 1. A lengthy, appended exposition of a topic or point. 2. A digression. n , pl -suses or -sus an incidental digression from the main...
This is the same as others, just a verification.