BorelSet - crowlogic/arb4j GitHub Wiki

In a topological space, a Borel set is any set that can be formed from open sets (or, equivalently, closed sets) through the operations of countable union, countable intersection, and relative complement. In the context of the real numbers, which are equipped with the standard topology, Borel sets include all open intervals, closed intervals, finite sets, countable sets, and many other complicated sets.