In general a term applied to the amount of some type of particle or energy crossing a unit area per unit of time. Magnetic energy is one type of flux. It is measured in Webers, or, as in the case of capturing and storing information as magnetic charges on tape; we use smaller units known as nanowebers (a nanoweber is one billionth of a Weber). Sources of magnetism create what we call a “magnetic field,” which is made up of lines of flux that may radiate in several or many directions, but are polarized according to the magnetic charge (polarity) of the source.
Flux is also a substance that can be placed on a metal being soldered or welded to help lower the melting temperature and/or prevent surface oxidation that can impede the process. It can also promote the fusion of metals, particularly in the welding process.