Coniunctio. Literally, "conjunction," used in alchemy to refer to chemical combinations; psychologically, it points to the union of opposites and the birth of new possibilities.
The coniunctio is an a priori image that occupies a prominent place in the history of man's mental development. If we trace this idea back we find it has two sources in alchemy, one Christian, the other pagan. The Christian source is unmistakably the doctrine of Christ and the Church, sponsus and sponsa, where Christ takes the role of Sol and the Church that of Luna. The pagan source is on the one hand the hieros-gamos, on the other the marital union of the mystic with God.[The Psychology of the Transference," CW 16, par. 355.]
Other alchemical terms used by Jung with a near-equivalent psychological meaning include unio mystica (mystic or sacred marriage), coincidentia oppositorum (coincidence of opposites), complexio oppositorum (the opposites embodied in a single image) unus mundus (one world) and Philosophers' Stone.