The rate of flow of fluid, particles, or energy through a given surface.
In terms of diffusuion flux is a measure of the rate of flow of particles from an area of high concentration to an area of low concentration.
J = P * ( [C]1 - [C]2 ), where J = Flux, P = permiability, [C]1 = high concentration, [C]2 = low concentration.
Flux is a measure of the amount of flow per unit time (e.g. grams per minute g.min-1, grams per second g.s-1)