Given [[Set|sets]] $A$ and $B$, the common part of $A$ and $B$ is called the **intersection** of $A$ and $B$ and is denoted by $A \cap B$. $ A \cap B=\{x \mid x \in A \text { and } x \in B\} $ When [[Set|sets]] $A$ and $B$ have no [[Element of a Set|elements]] in common, their intersection is the [[empty set]], and they are called [[disjoint]]