Definition:Countability Axioms

Definitions
Countability axioms is the common name used to refer to a set of properties of a topological space which have to do with the existence of countable sets, or countable families of open sets, satisfying certain conditions. They are not axioms in the strict sense of the word, but they are usually named as such because one may think of them as additional basic properties that one can ask from a topological space.

Euclidean space $\R^N$ satisfies all of the axioms below, and the same happens for many usual spaces, so they are properties that one may rely upon in many situations.

Also see

 * Axiom of countability at Wikipedia.