Unus mundus, Latin for "one world", is the concept of an underlying unified reality from which everything emerges and to which everything returns.
The idea was popularized in the 20th century by the Swiss psychoanalyst Carl Gustav Jung, though the term can be traced back to scholastics such as Duns Scotus and was taken up again in the 16th century by Gerhard Dorn, a student of the famous alchemist Paracelsus.
Jung and Pauli
Jung, in conjunction with the physicist Wolfgang Pauli, explored the possibility that his concepts of the archetype and synchronicity might be related to the unus mundus - the archetype being an expression of unus mundus; synchronicity, or "meaningful coincidence", being made possible by the fact that both the observer and connected phenomenon ultimately stem from the same source, the unus mundus.
Jung was always careful, however, to stress the tentative and provisional nature of such explorations into a unitarian idea of reality.