Flux represents a chaotic overflow of magic. It can be produced by abuse of various magical devices, and it is also the end-stage of the corruption which Taint inflicts upon logs of wood. Flux comes in two forms, gaseous ("flux gas") and liquid ("flux goo"). Both will inflict the Taint status effect on contact, but the gas flows upwards while the liquid flows downwards. Like other fluds, both can also flow sideways to seek a hole, but unlike water or lava, they have constant volume, and do not expand indefinitely.
If flux gas reaches the open sky, it will dissipate harmlessly into the void, but (at least in the overworld), flux goo lacks that option. Both will dissipate eventually regardless, but if they remain in contact with a block for too long, they can spawn Fibrous Taint, thus tainting not only the block, but its entire column from bedrock to sky.