Definition.
Synchronicity is the meaningful coincidence of events in the psyche (such as dreams or ideas of strong interest) with events in the outer world. In order to be a true synchronicity, these events cannot be causally connected.
For example, to dream of a rainbow-striped van, and then to see a rainbow-striped van outside a prospective employer’s office might be understood as a synchronicity. To go online after the dream in search of images of rainbow vans, and to successfully find one, probably would not be considered synchronous, because the act of searching online had a causal role in encountering the image.