Definition:
“A parenthetical element is information that is nonessential to the meaning of a sentence, such as an example, a clarification, or an aside. This type of sentence component may include the following types of clauses and phrases, as long as the information is nonrestrictive:
Relative clauses, which commonly begin with which, who/whom/whose, where, or when
Appositives, or nouns or phrases that rename preceding nouns or phrases
Participial phrases, or verb-based phrases that describe preceding nouns
Prepositional phrases, or preposition-based phrases that often describe preceding nouns
Phrases beginning with such as, including, e.g., or i.e.
The supplementary information provided by a parenthetical element is typically enclosed by two commas, parentheses, or dashes, separating the nonessential material from the rest of the text. Although these punctuation marks have a common purpose, the content of their associated text and their level of emphasis of this text may vary.”
“Editing Tip: Parenthetical Elements.” AJE, https://www.aje.com/arc/editing-tip-parenthetical-elements/. Accessed 6 September 2022.
Informal Definition:
Words or descriptors that aren’t necessary for the information in the sentence.
An example of a parenthetical element--
I did my homework yesterday, which was due this morning.
The bolded segment represents the part of the sentence, or the parenthetical element, that was not crucial to the main information being presented to the reader
Entomology:
The word parenthesis means "words, clauses, etc. inserted into a sentence, not grammatically connected to it but explaining or qualifying a word," is known from French parenthèse or directly from Medieval Latin
“parenthetical | Etymology, origin and meaning of parenthetical by etymonline.” Etymonline, 15 January 2020, https://www.etymonline.com/word/parenthetical. Accessed 6 September 2022.