When something happens by chance at the same time as something else, without being planned or caused by each other.
From Latin 'coincidentia' (falling together), combining 'co-' (together) + 'incidere' (to fall). Entered English in the 1600s-1700s to describe events that occur together by chance rather than design.
Humans are pattern-recognition machines, which is why we find coincidences so spooky—our brains are actually misinterpreting probability, and the word 'coincidental' is us trying to calm ourselves by labeling a frightening coincidence as 'just chance,' when really we're bad at statistics.
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