Coincidence is the noteworthy alignment of two or more events or circumstances without obvious causal connection. The word is derived from the Latin co- ("in", "with", "together") and incidere ("to fall on").

The index of coincidence can be used to analyze whether two events are related. A coincidence does not prove a relationship, but related events may be expected to have a higher index of coincidence. From a statistical perspective, coincidences are inevitable and often less remarkable than they may appear intuitively. The odds that two people share a birthday, for example, reaches 50% with a group of just 23 (see the Birthday problem).

In The Psychology of the Psychic the author David Marks describes four distinct meanings of the term "coincidence". Marks suggests that coincidences occur because of "odd matches" when two events A and B are perceived to contain a similarity of some kind. For example, dreaming of a plane crash (event A) would be matched by seeing a news report of a plane crash on the next morning (event B).

In optics, coincidence is also used to refer to two or more incident beams of light that strike the same point at the same time.

Remarkable coincidences sometimes lead to claims of psychic phenomena or conspiracy theories. Some researchers (see Charles Fort and Carl Jung) have compiled thousands of accounts of coincidences and other supposedly anomalous phenomena (see synchronicity). The perception of coincidences often leads to occult or paranormal claims. It may also lead to a belief in fatalism, that events are pre-destined to happen in the exact manner of a prior plan or formula. This lends certain events an aura of inevitability.

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