第二可数空间是指有一个可数基的拓扑空间，我们也将“具备可数基”这一性质当作一条公理（第二可数性公理）放在第二可数空间的定义中（与“有限交，任意并”一同）。

In topology, a second-countable space, also called a completely separable space, is a topological space satisfying the second axiom of countability. A space is said to be second-countable if its topology has a countable base. More explicitly, this means that a topological space is second countable if there exists some countable collection of open subsets of such that any open subset of can be written as a union of elements of some subfamily of . Like other countability axioms, the property of being second-countable restricts the number of open sets that a space can have.