parenthesis

noun, plural pa·ren·the·ses [puh–ren-thuh-seez] /pəˈrɛn θəˌsiz/.

1. 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.
2. Usually parentheses. the material contained within these marks.
3. 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.
4. an interval.

noun plural -ses (-ˌsiːz)

1. a phrase, often explanatory or qualifying, inserted into a passage with which it is not grammatically connected, and marked off by brackets, dashes, etc
2. Also called: bracket either of a pair of characters, (), used to enclose such a phrase or as a sign of aggregation in mathematical or logical expressions
3. an intervening occurrence; interlude; interval
4. in parenthesis inserted as a parenthesis

n.1540s, “words, clauses, etc. inserted into a sentence,” from Middle French parenthèse (15c.), from Late Latin parenthesis “addition of a letter to a syllable in a word,” from Greek parenthesis, literally “a putting in beside,” from parentithenai “put in beside,” from para- “beside” (see para- (1)) + en- “in” + tithenai “put, place,” from PIE root *dhe- “to put, to do” (see factitious). Sense extension by 1715 from the inserted words to the curved brackets that indicate the words inserted. A wooden parenthesis; the pillory. An iron parenthesis; a prison. [“Dictionary of Buckish Slang, University Wit and Pickpocket Eloquence,” London, 1811]