A flux is a material that lowers the melting temperature of another material.
Kiln-formed glass artists use flux to reduce devitrification. Hot glass artists, such as glass blowers, include a flux within their glass batch to reduce the cost of melting.
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.