either or both of a pair of signs () used in writing to mark off an interjected explanatory or qualifying remark, to indicate separate groupings of symbols in mathematics and symbolic logic, etc.
Usually, parentheses. the material contained within these marks.
Grammar. a qualifying, explanatory, or appositive word, phrase, clause, or sentence that interrupts a syntactic construction without otherwise affecting it, having often a characteristic intonation and indicated in writing by commas, parentheses, or dashes, as in William Smith—you must know him—is coming tonight.
a phrase, often explanatory or qualifying, inserted into a passage with which it is not grammatically connected, and marked off by brackets, dashes, etc