parenthetic

That nearly 100-year-old publication instructs sentence drafters to “enclose parenthetic expressions between commas” and to “place a comma before and or but introducing an independent clause.”

From Slate ● Jan. 28, 2014

Sir: The clue to their success is tucked unobtrusively into one parenthetic expression that you use: "Russian is more precisely phonetic than English."

From Time Magazine Archive

The position of an adverb is, in respect to matters of syntax, pre-eminently parenthetic; i.e., it may be omitted without injuring the construction.

From A Handbook of the English Language by R. G. (Robert Gordon) Latham

Returning from this parenthetic remark, we are concerned here chiefly to remember that, as said at the outset, there existed thirty years ago, no tenable theory about the genesis of living things.

From Essays: Scientific, Political, & Speculative, Vol. I by Herbert Spencer

Miss Bates, indeed, is verbose, roundabout, and parenthetic; but the widow never deviates into coherence.

From Dickens English Men of Letters by Sir Adolphus William Ward