Definition of Tethys
1 Greek Mythology a goddess of the sea, daughter of Uranus (Heaven) and Gaia (Earth). 2 Astronomy a satellite of Saturn, the ninth closest to the planet, discovered by Cassini in 1684. It is probably composed mainly of ice and has a diameter of 659 miles (1,060 km). 3 Geology an ocean formerly separating the supercontinents of Gondwana and Laurasia, the forerunner of the present-day Mediterranean.