Synchronicity is the experience of two or more events that are apparently causally unrelated or unlikely to occur together by chance, yet are experienced as occurring together in a meaningful manner. The concept of synchronicity was first described in this terminology by Carl Gustav Jung, a Swiss psychologist, in the 1920s.
The concept does not question, or compete with, the notion of causality. Instead, it maintains that just as events may be grouped by cause, they may also be grouped by meaning. A grouping of events by meaning need not have an explanation in terms of cause and effect.
Excerpt from Man & His Symbols (Audiobook) by Carl G. Jung on the concept of synchronicity. Thanks to JungianMaps on Youtube for the upload!