noun

• (rhetoric) A digression; the use of such digressions.
  "Mr. Trevanion was one of those talkers, who are too much engrossed with their own subject matter to have much attention to bestow elsewhere; with them silence is attention. Ethel's wandering eye, and lip, tremulous with its effort to speak, would never have attracted his notice. To his utter astonishment, she interrupted a parenthesis, as brilliant as the rocket which it depicted, by saying,— "Mr. Trevanion, I do not know what you will think of my boldness, but I must speak to you.""
• (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.