Soldering flux is a material, generally a gel or rosin in contemporary metalworking, that removes metal oxides from joints during metal soldering. Without flux, solder won't sufficiently wet the joints, resulting in weaker structural bonds.
A similar process can be used in other kinds of metallurgy, including extraction, welding, and brazing. The term comes from the Latin word "fluxus," which means flow. Early forms of flux include charcoal, lime, soda carbonate, and borax.