Parenthesis

A word, phrase, or sentence, by way of comment or explanation, inserted in, or attached to, a sentence which would be grammatically complete without it. It is usually inclosed within curved lines (see def. 2 below), or dashes.

One of the curved lines () which inclose a parenthetic word or phrase.

a message that departs from the main subject

either of two punctuation marks ‘(’ or ‘)’ used to enclose textual material