Synchronicity: His way of describing otherwise inexplicable or highly improbable coincidences involving human consciousness. For example, thinking of a very rare bird in its exacts colours coming through your window moments before the bird in fact comes through that window. The sense of premonition. Having a sense that so and so whom I haven't seen in a while is coming through my front yard and five minutes later so and so does.
Jung's explanation was interesting. He was convinced there was a shared level of consciousness among humans, he called it the collective unconscious, which coincides with Eastern notions of a shared universal self underlying our own reality. His main reasons were archetypes he discovered in human consciousness, standard symbols or motifs that are more or less permanent in our mind, or part of the structure of our mind. the stuff that appears universally in fairy tales/folk tales/myths or dreams all over human cultures; the good mother, the evil step mother, the trickster (like the fox or brother rabbit), the wise wizard/old man/guardian, a sense of journeying, or seeking a home/promised land, or being in wandering/wilderness, and so many more. The best way to understand them is a sort of innate "map" on what it means to be human, or the most basic aspects of human experience that seem already known/mapped in the human psyche from the get go, rather than created through personal experience. so meeting things that trigger the archetype can cause an instinctive attraction or repulsion to us. The great works of art, visual and musical, those that move deeply are said to be able to trigger the archetypes which can be seen (images) felt (emotion/feeling) thought (like aspects/rules of logic) or trigger sensual experience. So enduring works of art are able to capture archetypes. Encountering an archetype has a sense of "recognition" or even deja vu to us, like its something someway somehow known, even if foreign. That means we have mapped something onto an archetype/triggered an archetype. Jung believed they were part of a "collective unconscious" all humans have access to/or a shared consciousness.
The syncronicity also tends to be more likely/frequent the more individuated a person gets. That is, the more "whole" or psychically healthy a person grows, into himself, possessing more and more of his unconscious aspects (the personal unconscious, that is, not Jung's much more speculative "collective unconscious") into his integral/self-actuated self, the more these "coincidences" seem likely or perhaps the more penetrative (into outer reality) a person's consciousness gets.
I disagree with the collective unconsciousness business because I think we are in our core, individuals, based on the fact that no matter how much can be shared between us or between us and the cosmos, we cannot interpenetrate each other beyond a certain level. Somethings we can only know about others when they themselves share it, and somethings of ours can only be made available to others when we open ourselves up willingly. And yet even by will, much remains only our own, unshareable. So human experience I believe supports the basic intuition of individuality. I look at the archetypes as inherited or somehow transmitted rather than belonging to some universal consciousness that we all have together.
Yet there is also a cosmic aspect to us, or an unknown way in which we are somehow connected both to each other and to the cosmos/our field of existence, connected beyond our physically discernible bodies. I say discernible because while I lean towards a more spiritual explanation (our souls are much more vast than our bodies and may possibly be able sometimes to communicate with each other or to connect to the cosmos on a much wider field than we may presume or we can know) this is because I already accept the existence of the soul, but it need not necessarily be the only explanation for the synchronicity/archetypes. There may possibly be an energy around us that reaches much further than our physical bodies, that may not yet be discernible scientifically but could possibly be. That is, there may possibly be more to our physical selves, than we discern. A soul, in its spiritual aspects a least, would always lie beyond any such discernment, even theoretically.
Just musings of mine after a simple convo that reminded me of my old pal, Carl Jung. Enjoy! (those who be interested).