The term synchronicity is coined by Jung to express a concept that belongs to him. It is about acausal connection of two or more psycho-physic phenomena. This concept was inspired to him by a patient's case that was in situation of impasse in treatment. Her exaggerate rationalism (animus inflation) was holding her back from assimilating unconscious materials. One night, the patient dreamt a golden scarab - cetonia aurata. The next day, during the psychotherapy session, a real insect this time, hit against the Jung's cabinet window. Jung caught it and discovered surprisingly that it was a golden scarab; a very rare presence for that climate.
So, the idea is all about coincidence: in this case, between the scarab dreamt by the patient and its appearance in reality. But this coincidence is not senseless, a simple coincidence. By using the amplification method, Jung associates in connection with the scarab and comes to the concept of death and rebirth from the esoteric philosophy of antiquity, a process that, in a symbolic way, the patient should experience for a renewal and vitalization of her unilateral personality, the cause of the neurosis she was suffering from.
Thus, a significant coincidence of physical and psychological phenomena that are acausal connected. Behind all these phenomena Jung places the archetype or the constellation of an archetype, which, in his view, is a process that engages equally objective manifestations, in the physical world, and subjective ones, in the psychological universe.