A topological space is second countable if it has a countable topological basis.
Second Countable Topology
See also
Topological Basis, Topological SpaceExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Second Countable Topology." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/SecondCountableTopology.html