Synchronicity is when two things happen at the same moment and feel connected, even though neither one caused the other. You think of an old friend, and that afternoon a letter from her arrives. Nothing made that happen — but it does not feel like nothing, and that feeling is what the word names. The psychologist Carl Jung invented it for exactly this: meaningful coincidence. So the word carries a hint of the mysterious, and people reach for it when a coincidence seems too neat to be only a coincidence. Careful speakers keep it separate from plain 'coincidence' (no meaning attached) and from 'synchronisation' (things deliberately made to match). But be warned that people do use it loosely, especially in business and sport, to mean nothing more than good timing or a team working smoothly together — 'the synchronicity of the two dancers'. A strict teacher would call that a misuse; it is common enough now that you will meet it, so read the whole sentence before you decide which one someone means.