The sum of two or more random variables.

Some families of distributions are closed under sums. For instance, the sum of two independent Poisson variables is itself Poisson distributed, and the sum of two normally distributed variables is again normally distributed.

Sums of other distributions are still tractable. For instance, the sum of $r$ independent identical geometric distributions has a negative binomial distribution.

Finally, some distributions' sums can only be approximated, e.g., the sum of two lognormal distributions.