What is "sidero"

In Greek mythology, Sidero (, "the Iron One") was the second wife of Salmoneus and stepmother of Tyro. Sidero mistreated Tyro. When Pelias and Neleus, Tyro's twin sons, reached adulthood they sought revenge. Although Sidero fled from them into Hera's district, Pelias nevertheless murdered her atop the altar to Hera.