The rate at which any physical quantity is transported (usually in a hemisphere of directions) across a unit area. The term “flux” indicates flow (rate of transport), and “density” here is an areal (as opposed to volumetric) density. Flux densities often may be written in the form pnv, where n is the number density (volumetric) of carriers of a property p (e.g., mass, charge, momentum, energy) and v is their speed.