countably

Example:The set of all integers is countably infinite because you can list them as: 0, 1, -1, 2, -2, 3, -3, and so on.

Definition:A set with the same cardinality as the natural numbers. It's a countably infinite set, and while you can't write down every element, you can list them in a sequence where each element corresponds to a natural number.

Example:The Lebesgue measure on the real line is countably additive, which means that the measure of a countable union of pairwise disjoint intervals is the sum of their individual lengths.

Definition:In measure theory, a measure is countably additive if the measure of a countable union of pairwise disjoint sets is the sum of the measures of the individual sets.