countably additive set function

countably additive set function

[¦kau̇n·tə·blē ¦ad·əd·iv ′set ‚fəŋk·shən]
(mathematics)
A set function with the properties that (1) the union of any finite or countable collection of sets in the range of the function is also in this range, and (2) the value of the function at the union of a finite or countable collection of sets that are in the range of the set function and are pairwise disjoint is equal to the sum of the values at each set in the collection. Also known as completely additive set function.