Parenthesis

In rhetoric, a parenthesis (plural: parentheses; from the Greek word παρενθεσις) is (according to the Oxford English Dictionary)

"An explanatory or qualifying word, clause, or sentence inserted into a passage with which it has not necessarily any grammatical connexion, and from which it is usually marked off by round or square brackets, dashes, or commas."

By extension, the word parentheses, seldom used in the singular, has come to refer to the round brackets in which a parenthesis is enclosed in writing. See bracket for an account of these punctuation marks.