1911 Encyclopædia Britannica/Parenthesis

PARENTHESIS (from Gr. παρεντιθέναι, put in alongside), the grammatical term denoting the insertion (and so also the signs for such insertion) of a word, phrase or sentence between other words or in another sentence, without interfering with the construction, and serving a qualifying, explanatory or supplementary purpose. In writing or printing such parenthetical words or sentences are marked off by commas, dashes, or, more usually, by square or semi-circular brackets.