(grammar) A countable noun.

Capable of being counted; having a quantity. / (mathematics, of a set) finite or countably infinite; having a one-to-one correspondence (bijection) with a subset of the natural numbers.

countability

plural of countable

In a countable manner; in a way that can be counted. / (mathematics) In a way that can be put into one-to-one correspondence with the natural numbers or any subset thereof.

The quality of being countable.

(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.

Alternative form of second-countable

(set theory) A set that is countable.

(topology) Such that its topology has a countable base, said of a topological space.