Question

What is a Flux ???

Answer 1

Flux is a continuous or a non-stop flow of substances in a system.

Answer 2

Flux is used in Physics in a rather inconsistent way. However it has and underlying characteristics referring to the amount of some physical quantity that is passing through a surface. For example electric field flux. It can also be interpreted as the total amount of flow of some quantity say photons passing through a given area, i.e., Photon flux but this term is archaic and today flow of particles are usually referred to as the Fluence in the description of such beams.

Another term Flux Density is sometimes used and refers to the amount of some physical quantity per unit area as for example magnetic flux density specified by B.