(mathematical analysis) The following property of a measure: that the measure of a countable union of disjoint sets is equal to the sum of their measures.

(mathematical analysis) Of a measure: that satisfies countable additivity.

plural of countable set

(set theory) A weaker form of the axiom of choice that states that every countable collection of nonempty sets must have a choice function; equivalently, the statement that the direct product of a countable collection of nonempty sets is nonempty.