The terms Synchronicity and the Synchronistic Principle were introduced by Carl Gustav Jung.

in the 1930s, Jung devoted himself to the study of the I Ging and stated that this science is not founded on a pricinple of causality but by a yet to be named princinple.
He chose the term Synchronistic to describe events that are not causally linked, are correlated in time and connected with each other and can be perceived and interpreted as interrelated.

In synchronicity, an inner event (idea, dream, vision, emotion) and an external event coincide where the later is a manifestation of the inner state. The internal event must happen with the outer chronologically before or at the same time (synchronously). Otherwise, it would be a response to the external event which indicates a causal link.
Jung considered synchronicity to contain a creative, transformative potential. Synchronicity is an addition to causality and neither negates nor attenuates it.