Plain-language answer
A coincidence is the notable co-occurrence, or close succession, of events for which no relevant causal connection is known or established. Two things happen, close together in time, space, or resemblance; nothing links them that anyone can point to; and a person notices.
That definition is doing more work than it looks like. It bundles together several separate questions that are worth pulling apart before you try to explain any particular coincidence.
Why it matters
Most arguments about a coincidence are really arguments about which of its components is in dispute. Someone says “that can’t be chance” and means the improbability feels too large. Someone else says “it clearly means something” and is making a claim about personal significance, not about cause. A third person says “there must be a connection” and is making an empirical claim that needs evidence, not just a feeling that there should be one.
Keeping these apart is the single most useful habit this site can teach, and it follows statisticians who have argued that studying coincidences rigorously means separating the data-gathering question from the modelling question from the question of what, if anything, the result establishes.(Diaconis & Mosteller, 1989)
Five things a coincidence claim can mean
Coincidence as an event relationship. At its most basic, a coincidence is just a fact about the world: two or more events occurred together in a way that stands out against the background of everything else that didn’t.
Surprise as a psychological response. Surprise is what happens in you, not in the event. It is shaped by how memorable, recent, or personally relevant the events are — which is why two people can witness the same co-occurrence and find it strikingly different in how surprising it feels.
Improbability under a specified model. A probability is never a bare fact about an event; see Probability is a model, not a verdict. “How unlikely was this?” only has an answer once you say unlikely under which assumptions, counting which opportunities.
Meaningfulness as a personal or cultural interpretation. People are entitled to find an event meaningful — as a turning point, a comfort, a story worth telling — independent of any claim about its probability or cause. This site treats that meaning as real without treating it as evidence for a mechanism.
Causal independence as a claim requiring evidence. Saying two events are causally unconnected is itself a claim, not a default. Sometimes an apparent coincidence turns out to share a common cause (a shared source, a seasonal pattern, a social network) that simply hadn’t been noticed yet — finding that out is empirical work, not a matter of definition.
Common misconception
“It was just a coincidence” and “it was really meaningful to me” are often treated as if they contradict each other. They don’t have to: the first is a claim about probability and cause, the second is a claim about personal significance. A low-probability event under a crude model can still be emotionally real and worth reflecting on; that does not require it to be caused by anything beyond chance.
Limits and open questions
“No relevant causal connection is known or established” is a statement about the current state of knowledge, not a metaphysical guarantee. Some co-occurrences that look like pure coincidence today reflect a shared cause that simply hasn’t been identified — a common seasonal driver, an unnoticed network link, a reporting bias. Calling something a coincidence is the start of an inquiry, not the end of one.
Related
- The taxonomy sorts coincidences into descriptive categories — temporal, spatial, resemblance, and more — each with its own analytical question.
- Probability is a model, not a verdict goes deeper on the “improbability” component above.
Key takeaways
- A coincidence claim usually bundles five separable things: the event, the surprise it causes, its improbability under some model, its personal meaning, and a claim about causal independence.
- Disagreements about a coincidence are often disagreements about just one of these components, mistaken for a disagreement about all of them.
- “No known causal connection” describes the current evidence, not a permanent fact about the event.