Parenthesis

Part of speech: noun (n.)

1. 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. "Seldom mentioned without a derogatory parenthesis." Sir T. Browne. Don't suffer every occasional thought to carry you away into a long parenthesis. Watts.

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