flux is a concept used often in the field of
vector calculus. It can be described in simple terms as the "amount" of a
vector field that passes through a certain
surface area. In mathematical terms, the flux F is defined as the integral of the dot product of the vector field
F and the surface normal
N over the surface, like so:
F = ∫s N · F ds
Flux is used a lot in engineering to describing the flows of various things such as fluids (liquids and gases), and the magnetic field inside of transformers.
The flux is intimately tied to the concept of divergence, the infinitesimal limit of the ratio of flux through a closed surface to the volume enclosed by that surface.