The Borel 
-algebra
on a topological space is the sigma-algebra
generated by its open sets (or equivalently, by its closed sets).
Borel Sigma-Algebra
See also
Borel Measure, Sigma-AlgebraExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Borel Sigma-Algebra." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/BorelSigma-Algebra.html