> [!definition] > > Let $X$ be a [[Set|set]]. Then $\cm \subseteq \pow{X}$ is an **algebra** if it is [[Closure|closed]] under complement and finite intersections: > $ > A \in \cm \Rightarrow A^c \in \cm \quad A, B \in \cm \Rightarrow A \cup B \in \cm > $ > > $\cm$ is also closed under finite intersections.