Editing Probability density function (section)

===Discussion===
In the [[#Absolutely continuous univariate distributions|continuous univariate case above]], the reference measure is the [[Lebesgue measure]]. The [[probability mass function]] of a [[discrete random variable]] is the density with respect to the [[counting measure]] over the sample space (usually the set of [[integer]]s, or some subset thereof).

It is not possible to define a density with reference to an arbitrary measure (e.g. one can not choose the counting measure as a reference for a continuous random variable). Furthermore, when it does exist, the density is almost unique, meaning that any two such densities coincide [[almost everywhere]].