Meaning of sign of aggregation

sign' of aggrega'tion

Pronunciation: [key]
— Math. Math.
  1. any of the signs used to indicate grouping in an algebraic expression: vinculum, bar, or raised horizontal line, &aspplusspbhorbarover;; a pair of parentheses, (a + b); a pair of brackets,
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.