Synchronicity is a word coined by the esteemed physiologist Carl Jung who discovered the concept. That doesn't mean he invented synchronicity. He uncovered it. Synchronicity has always been in existence. Jung said, among other things on the subject, that synchronicity was "a principle of the universe that is acausal." A principle of the universe. By that he was not referring to the seen universe alone. He was talking about the unseen reality and its interplay with the unconscious mind (Carl Jung also uncovered and deeply explored the concept of the unconscious mind).
The average person thinks of synchronicity as simple coincidence - or a coincidence that is eerie and feels too ironic to only be an accident. Indeed, if an occurrence feels too ironic to be an accident, it is not an accident - it is true synchronicity. So what is true synchronicity?
Synchronicity is meaningful coincidence occurring within an improbable coinciding of events. Synchronicity is not only a coinciding of events but is a coinciding of realities - a colliding of the seen and unseen realities. Within the improbable events there will be layers of hidden meanings that ring true in your innermost being.
For synchronicity to truly be a connecting principle of the multiverse, seen and unseen, it must deeply involve the nonlinear side of life. You all experience the nonlinear aspect of existence when you dream at night. Your dream states are nonlinear experiences of consciousness. So, you have this reference for the nonlinear reality in your own experience of dream states. Synchronicity too is nonlinear just like your dreaming mind. It makes use of data from beyond space and time, things that seem to be outside of time, things that seem to come from the past or the future. So, synchronicity comes from the nonlinear unseen universe and it that way is very mysterious and magical. It is more than your linear reality can logically explain. Right? This asks of you - acceptance.
Consider that reality is multidimensional. Science is exploring more of this notion all the "time." Now consider the notion that the whole grand interwoven net of multidimensional reality comes together at intersecting points - points beyond space and time touching points in space and time. These intersections are gateway events, synchronicities, meaningful incidents that appear as simple coincidences. The ancient Hindu mystics, or seers, said everything in the universe is inextricable connected. They used the image of a multidirectional net which they called "Indra´s Net" to illustrate the concept. If the net is multidimensional the points where the strings of the net connect would be like intersecting points from which one could access the whole net. One tug pulls the whole net, one tug connects you to the whole net! Basically that is how synchronicity works.