Symmetric Difference

The symmetric difference $A \triangle B$ is the set of elements in $A$ or $B$ but not both.

You can compute it as $A \triangle B = (A \setminus B) \cup (B \setminus A)$ or as $(A \cup B) \setminus (A \cap B)$.

For example, if $A = \{1, 2, 3\}$ and $B = \{2, 3, 4\}$, then $A \triangle B = \{1, 4\}$. You exclude $2$ and $3$ because they appear in both.