👉 Borel is a mathematical concept in set theory, which refers to the collection of all subsets of a given set. It is named after the French mathematician Émile Borel who first introduced it in 1905. The Borel definition of a subset of a set \(A\) can be stated as follows: Definition: Let \(S \subseteq A\). Then, \(S\) is called a borel subset of \(A\) if and only if for every