Definition (Sum set / difference set / etc). Let $G$ be an abelian group and let $A,B\subset G$.

The sumset $A+B$ is the set $\{x+y:x\in A,y\in B\}$.

The difference set $A-B$ is the set $\{x-y:x\in A,y\in B\}$.

We write $2A$ for $A+A$, $3A$ for $A+A+A$, etc.