Mnemosyne (Greek Mνημοσυνη, pronounced: ni-ma-zi-ne in 4 syllables, and not to rhyme with sign) was the personification of memory in Greek mythology. This titaness was the daughter of Gaia and Uranus and the mother of the Muses by Zeus. The word mnemonic is derived from her.
Mnemosyne was also the name for a river in Hades, counterpart to Lethe, according to certain gnostic groups in ancient Greece. Dead souls drank from Lethe so they would not remember their past lives when reincarnated. Initiates were encouraged to drink from the river Mnemosyne when they died, instead of Lethe.