Caption: Generic flux diagram for general flux vector F, through a general differential vector surface element. Vector n is the unit vector normal to the surface element.

Speaking loosely, flux is flow rate of something per unit area through a surface.

Note also that flux and intensity can mean the same thing depending on context and it's easy to flip back and forth between them when lecturing.

Actually, the term is generalized meaning a vector field dot producted with the unit vector normal to the differential surface element integrated over some surface region.

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