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