parenthesis

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. 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.
2. n. 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.
3. n. A comment departing from the theme of discourse; a digression.
4. n. An interruption of continuity; an interval: "This is one of the things I wasn't prepared for—the amount of unfilled time, the long parentheses of nothing” ( Margaret Atwood).

Century Dictionary and Cyclopedia

1. n. 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. It is regularly included by two upright curves facing each other (also called parentheses), or the variant form of them called brackets, but frequently by dashes, and even by commas. The quotation from Dryden given below contains a parenthesis.
2. n. 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. The parentheses ( ), including the square form [ ] also called crotchets and now usually brackets, were formerly (as in the first quotation under def. 1) used to separate a word or words typographically, where quotation-marks are now used. In phonetic discussions (Ellis, Sweet, etc.) the curves are often used for a similar purpose, to indicate that the letters of the words so inclosed have a fixed phonetic value, according to a system previously explained. The curves are also used to inclose small marks and letters, and figures of reference, in order to make them more distinct to the eye.
3. n. An interval; a break; an episode.
4. n. Abbreviated par.

Wiktionary

1. n. 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.
2. n. Either of a pair of brackets, especially round brackets, ( and ) (used to enclose parenthetical material in a text).
3. n. rhetoric A digression; the use of such digressions.
4. n. 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.

GNU Webster's 1913

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

WordNet 3.0

1. n. either of two punctuation marks (or) used to enclose textual material
2. n. a message that departs from the main subject