parenthesis

Concise Oxford English Dictionary © 2008 Oxford University Press:
parenthesis /pəˈrɛnθɪsɪs/

▶noun (pl. parentheses /-siːz/)

• 1 a word or phrase inserted as an explanation or afterthought, in writing usually marked off by brackets, dashes, or commas.

• 2 (parentheses) a pair of round brackets () used to include such a word or phrase.

– derivatives
parenthesize (or parenthesise) verb.

– origin C16: via late L. from Gk, from parentithenai ‘put in beside’.