probability mass

The _number a discrete distribution assigns to one outcome_. For a fair die, `P(X = 3) = 1/6`; the probability mass at `3` is `1/6`. The function `pmf(x) = P(X = x)` is the _probability mass function_ — the whole distribution as a lookup table. The masses sum to 1 across all outcomes. Discrete distributions are made of masses; continuous distributions are made of _densities_. The distinction matters because at a single point of a continuous distribution the probability is _zero_; only the density is meaningful there.