In rhetoric, a parenthesis (plural: parentheses; from the Ancient Greek word παρένθεσις parénthesis 'injection, insertion', literally '(a) putting in beside') or parenthetical phrase is an explanatory or qualifying word, clause, or sentence inserted into a passage. The parenthesis could be left out and still form grammatically correct text. Parenthetical expressions are usually delimited by round or square brackets, dashes, or commas.
The following are examples of types of parenthetical phrases:
While a parenthesis need not be written enclosed by the curved brackets called parentheses, their use, principally around rhetorical parentheses, has made the punctuation marks the only common use for the term in most contexts.
English-language style and usage guides originating in the news industry of the twentieth century, such as the AP Stylebook, recommend against the use of square brackets for parenthesis and other purposes, because "They cannot be transmitted over news wires." Usage of parentheses goes back (at least) to the 15th century in English legal documents.