Thetis
in mythology, one of the sea deities, who was courted by Neptune and Jupiter. But when the gods were informed that the son she would bring forth must become greater than his father they ceased their solicitations; and Peleus, the son of Eacus, was permitted to gain her hand. Thetis became mother of several children, among them Achilles whom she rendered invulnerable by plunging him in the waters of the Styx, except that part of the heel by which she held him.