A series with two indices. For example, for any joint distribution with $p_{ij}=\text{Pr}\left\{X=x_i,Y=y_j\right\}$, $E\left(X+Y\right)={\displaystyle \sum _i{\displaystyle \sum _j\left(\left(x_i+y_j\right)\times p_{ij}\right)}}$.