Mnemosyne was one of the Titans, daughter of Uranus and Gaea, and goddess of memory. She was also occassionally referred to as Mneme; however, this was the name of another goddess. She was the oracular goddess of the underground oracle of Trophonios in the region of Boeotia.
Zeus slept with Mnemosyne for nine consecutive days, eventually leading to the birth of the nine Muses. In Hesiod's Theogony, the kings and poets were inspired by Mnemosyne and the Muses, thus getting their extraordinary abilities in speech and using powerful words.
The name Mnemosyne was also used for a river in the Underworld, Hades, which flowed parallel to the river of Lethe (which means forgetfulness). Usually, the souls of the dead would drink water from Lethe, so that they would forget their past lives when they would be reincarnated. However, the souls of the novices were told to drink water from Mnemosyne. This myth may have been part of a small mystic religion or be tied to Orphic poetry.