A flux is a material that lowers the melting temperature of another material.
Stained glass artists use flux to lower the melting temperature of the solder. Kiln-formed glass artists use flux to reduce devitrification. Hot glass artists, such as glass blowers, include a flux within their glass batch.
Borax is one example of a flux. Soda (or potash) is a common flux used for glass and is one of the three primary ingredients (along with silica/sand and lime) in soda lime glass.
Editor's Note: I've updated this GlassFact to remove the use of the word overspray to describe a flux solution that is used to prevent devitrification. Thanks to my friends at Bullseye Glass for pointing out that the correct term is overglaze.