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.
Origin: 1560–70; < LL < Gk parénthesis a putting in beside. See par-, en-2, thesis
pa·ren·the·sis (pə-rěn'thĭ-sĭs) n.
pl.par·en·the·ses (-sēz')
Either or both of the upright curved lines, ( ), used to mark off explanatory or qualifying remarks in writing or printing or enclose a sum, product, or other expression considered or treated as a collective entity in a mathematical operation.
A qualifying or amplifying word, phrase, or sentence inserted within written matter in such a way as to be independent of the surrounding grammatical structure.
A comment departing from the theme of discourse; a digression.
An interruption of continuity; an interval: "This is one of the things I wasn't prepared for—the amount of unfilled time, the long parentheses of nothing"(Margaret Atwood).
[Late Latin, insertion of a letter or syllable in a word, from Greek, from parentithenai, to insert : para-, beside; see para-1 + en-, in; see en in Indo-European roots + tithenai, to put; see dhē- in Indo-European roots.]
[puh-ren-thuh-sis] 
θə