A space which is isomorphic to a Borel subset 
of a Polish space equipped
with its sigma-algebra of Borel
sets.
Standard Space
See also
Borel Set, Polish Space, Sigma-AlgebraExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Standard Space." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/StandardSpace.html