= Probability mass function
{title2=$p_X(x)$}
{wiki}
For a <discrete probability distribution>, the <probability mass function> of its <random variable> $X$ is
$$
p_X(x)=\mathbb P(X=x),\qquad p_X(x)\geq0,\qquad\sum_x p_X(x)=1.
$$
The sum ranges over its finite or <countable set> of possible values. An event's probability is the sum of the masses of the included values. For the example under <probability distribution>, $\mathbb P(X\geq1)=p_X(1)+p_X(3)=0.8$. The <cumulative distribution function> sums masses below a threshold; its jump at a value equals that value's <probability mass>. This differs from a <probability density function>, whose height is not a point probability.
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