Synchronicity - the unexpected merging of two separate events that are emotionally and/or vibrationally connected with each other that come together in outer or inner reality. These events are brought about by vibrational affinity, not through one's own will.
Synchronicity can relate to people who appear suddenly just after one has thought about them; the answer to a problem one is trying to solve coming into view through some external analogy that discloses the solution; sounds, sights, words, or phrases that one hears on the radio or elsewhere that correspond to one's inner process of reflection; meeting long unseen friends by 'chance', etc.
Synchronicity is an outer manifestation of inner connection, demonstrating the relationship between all things that are part of each other in an interactive network of 'flow'. This network brings things together at meaningful times and allows the Divine plan to unfold according to its own purposes.
In a deep sense, all of life is based on synchronicity, since we are at all times emitting energetic impulses without the conscious self being aware of it, and these energetic impulses find their way into the spiritual matrix of which we are a part, drawing to us those people, places, things, and events that will interact most fully with the energy we are sending forth. Viewed in this light, all of life is 'synchronized' with our readiness to experience.
When things one hopes for do not appear, or when thoughts one deliberately sends forth do not produce the anticipated result, this does not mean that the energetic principle of synchronicity is failing. Always, this principle draws together what is meant to be together. However, we are not always aware of the other frequencies on which we are sending forth impulses into the world. Like a radio with multiple frequency bands, we may have conscious thoughts broadcasting on one band, and a wide variety of unconscious, preconscious, and superconscious thoughts broadcasting on others. Self-awareness and clarity of focus narrows the range of bands upon which one is unconsciously shaping the world.
The great mystery and beauty of life lies in this: that life's unfoldment is exquisitely geared to our readiness to receive it. For better or worse, we are the co-creators of the great Mystery that represents the shape of our life.