I had always assumed the Jedi in the title of Return of the Jedi referred to Luke Skywalker's return from defeat. But Jedi is its own plural -- could it be referring to the rebuilding of the organization/movement/religion that presumably occurs after the Empire is defeated? Or maybe even to the return of Anakin's good side?
Yes, the title was originally Revenge of the Jedi, but that doesn't make it much clearer. It could still be either Luke's revenge, or the Jedi's revenge over the Sith, or even something else.
Did Lucas ever say exactly what it was he intended?