parenthesis

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun 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.
• noun 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.
• noun A comment departing from the theme of discourse; a digression.
• noun An interruption of continuity; an interval.

from The Century Dictionary.

• noun An explanatory or qualifying clause, sentence, or paragraph inserted in another sentence or in the course of a longer passage, without being grammatically connected with it.
• noun The upright curves ( ) collectively, or either of them separately, used by printers and writers to mark off an interjected explanatory clause or qualifying remark: as, to place a word or clause in parenthesis or within parentheses.
• noun An interval; a break; an episode.
• noun Abbreviated par.

from the GNU version of the Collaborative International Dictionary of English.

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

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A clause, phrase or word which is inserted (usually for explanation or amplification) into a passage which is already grammatically complete, and usually marked off with brackets, commas or dashes.
• noun Either of a pair of brackets, especially round brackets, ( and ) (used to enclose parenthetical material in a text).
• noun rhetoric A digression; the use of such digressions.
• noun mathematics, logic Such brackets as used to clarify expressions by grouping those terms affected by a common operator, or to enclose the components of a vector or the elements of a matrix.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun either of two punctuation marks (or) used to enclose textual material
• noun a message that departs from the main subject