Uranus and Gaia and the consort of Cronos, her brother, to whom she bore Hestia, Demeter, Hera, Hades, Poseidon and Zeus. It is a pity that for such an important character we have such an uncertain name.
Attempts have been made to associate it with that of Hera, by means of a kind of transposition, as well as to derive it from rheo, 'to flow', this latter interpretation being made by the third century BC Greek philosopher Chrysippus ('Golden Horse'!) 'because rivers flow from the earth'.
Leonard Ashley sees the name, however, as meaning 'thingness', which is about as basic - or as vague - as one can get. She does really seem to have been an earth goddess, however, as the name of her Roman equivalent Ops suggests (see this name), so perhaps it is best to return to Hera and decide in favour of one of the origins we considered for her name - that of era, 'earth'.