Meaning of countably additive function

count'ably ad'ditive func'tion

Pronunciation: [key]
— Math. Math.
  1. a set function that upon operating on the union of a countable number of disjoint sets gives the same result as the sum of the functional values of each set. Cf. finitely additive function.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.