pa·ren·the·sis / pəˈren[unvoicedth]əsis/ • n. (pl. -ses / -ˌsēz/ ) a word, clause, or sentence inserted as an explanation or afterthought into a passage that is grammatically complete without it, in writing usually marked off by curved brackets, dashes, or commas. ∎  (usu. parentheses) one or both of a pair of marks ( ) used to include such a word, clause, or sentence. ∎  an interlude or interval: the three months of coalition government were a lamentable political parenthesis.PHRASES: in parenthesis as a digression or afterthought.ORIGIN: mid 16th cent.: via late Latin from Greek, from parentithenai ‘put in beside.’

PARENTHESIS [Stress: ‘pa-REN-the-sis’. Plural parentheses (‘-seez’)].
1. In GRAMMAR, a qualifying, explanatory, or appositional word, phrase, clause, or sentence that interrupts a construction without otherwise affecting it. A written or printed parenthesis may be marked by pairs of COMMAS, DASHES, or round BRACKETS/parentheses: Our new manager (he has just this minute arrived) would like to meet you. A spoken parenthesis has the same intonation as an aside.

2. In the plural, a name for round brackets: the general term in AmE, but a less common, more technical term in BrE (short form parens). See ACADEMIC USAGE, ASIDE.

parenthesis qualifying matter introduced into a passage XVI; device used to mark this, e.g. () XVIII. — late L. — Gr. parénthesis, f. parentithénai place in besides; see PARA-1, EN-2, THESIS.
So parenthetic(al) XVII. — medL.