the.com/magnetic flux

means The total amount of magnetic field passing through a given surface, calculated from the field strength, the area, and the angle between them.

from Built from 'magnetic' (see magnet, via Latin magnes, from Magnesia, the region the Greeks associated with lodestone) plus 'flux,' from Latin fluxus/fluere, 'to flow.' The 'flow' sense is not literal here — nothing actually streams through the surface — but 19th-century physicists like Faraday and Maxwell, working out field theory, pictured field lines streaming through a surface, and the name stuck to the measurable quantity of that imagined flow.