Flux is a concept in applied mathematics and vector calculus which has many applications to physics. It describes any effect that appears to pass or travel through a surface or substance, whether it actually moves or not. In physics, flux is a measure of the number of electric or magnetic field lines passing through a surface in a given amount of time. Flux is a vector quantity in transport phenomena, describing the magnitude and direction of the flow of a substance or property. In vector calculus, flux is a scalar quantity, defined as the surface integral of the perpendicular component of a vector field over a surface.

In addition to its use in physics, the term flux appears in other fields, including medicine (fluids passing through or from a body), fluid dynamics (the flow of water in a stream), metalwork (a substance used to bond metals during soldering or welding), computing (as both an open-source software used in developing artificial intelligence applications and in cybersecurity), common language (indicating variability), and entertainment (as a continuous change) .