#### Answer

"$\cup$" (union of sets)

#### Work Step by Step

Looking at the CardinaIity of a Union,
If A and $B$ are finite sets, then $n(A\cup B)=n(A)+n(B)-n(A\cap B)$.
If A and B are disjoint, their intersection is empty, so, in that case,
$n(A\cup B)=n(A)+n(B)$
implying that "$\cup$" (union) could represent "sum" of disjoint sets.