Complex sentence: In mathematics, a σ-algebra (or sigma-algebra) over a set "X" is a nonempty collection Σ of subsets of "X" (including "X" itself) that is closed under complementation and countable unions of its members.
Split into simpler sentences: In mathematics, a σ-algebra (or sigma-algebra) is a technical concept for a collection of sets satisfying certain properties. Formally, σ-algebra a over a set "X" is a nonempty collection Σ of subsets of "X" (including "X" itself) that is closed under complementation and countable unions of its members.