Parenthesis  n. (pl. Parentheses) 1 a explanatory or qualifying word, clause, or sentence inserted into a sentence etc., and usu. Marked off by brackets, dashes, or commas. B (in pl.) Round brackets ( ) used for this. 2 interlude or interval.  parenthetic adj. Parenthetically adv. [greek: related to *para-1, *en-2, *thesis]