Symposium, a philosophical work by Plato written between 385 and 370 BC, is about a friendly competition between speeches given by famous men at a banquet. During the talk, Socrates says that a priestess from Mantinea named Diotima taught him "the philosophy of love" when he was young. Socrates also says that Diotima slowed down the spread of the Plague of Athens, which destroyed the ancient Greek city-state of Athens in the second year of the Peloponnesian War (430 BC). Aside from these few details, we don't know much about Diotima as a person, until now.  

You can also read about Diotima in the History made Beautiful blog.