Second Countable Space
A second countable space is a topological space whose topology is second countable.
See also
Second Countable Topology, Topological SpaceThis entry contributed by Edward Welbourne (author's link)
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Welbourne, Edward. "Second Countable Space." From MathWorld--A Wolfram Resource, created by Eric W. Weisstein. https://mathworld.wolfram.com/SecondCountableSpace.html