To me the definition is: Synchronicity is the experience of two or more events that are causally unrelated occurring together in a
meaningful manner. Since the meaning can only be inferred by the observer, then synchronicity means different things to different people. Again, (I think this is the 5th time in this thread) it's more an emotion, not something quantifiable that can be measured. You guys keep arguing statistics and odds when that has very little to do with it. The odds could be 1 to 2 or 1 to 2,000,000,000 it doesn't matter, all that matters is how the observer
FELT about the outcome. I keep bringing up this comparison, but it's like deja vu. You can't prove deja vu or test for it, and neither can you prove synchronicity or test for it. It's just an odd feeling you get when you witness a coincidence, or experience a coincidence, and you infer meaning into it. This shouldn't be a debate with statisticians, it should be a debate with psychologists.
It's not apophenia, that is seeing patterns where none exist. You could say someone found patterns in a string of coincidences and they found meaning in it, but just plain apophenia is not it. It can be part of the definition, but not the definition itself.
Synchronicity is just a word coined for an emotion that no one had a name for. I know I'm just a lay person but I've read everything I could get my hands on about this topic since Sept 11, 2001 when a string of coincidences
saved my life. It's kinda frustrating seeing a continued argument over the odds of flipping a coin when that has nothing to do with it.