/ˌkəʊɪnˈsaɪd/
[verb] To happen at the same time as something else, or to be exactly the same as or in agreement with something. For example, two events coincide when they occur together, or two opinions coincide when they match perfectly.
At its heart, 'coincide' literally means two things 'falling together' — as if the universe dropped them into the same spot at once.
The story of 'coincide' begins in Latin, with two small but powerful building blocks. The first is the prefix 'co-', a shortened form of 'com-', meaning 'together' or 'jointly'. The second is the verb 'incidere', which meant 'to fall upon' or 'to happen', itself built from 'in-' (meaning 'on' or 'into') and 'cadere' (meaning 'to fall'). Put them together and you get 'coincidere' — literally 'to fall together onto the same point'. It is a beautifully physical image: two separate things dropping from different directions and landing in exactly the same place.
Latin 'cadere', meaning 'to fall', is one of the most productive roots in the entire English language. It gave us a remarkable family of words through different routes. Directly from Latin we get 'cadence' (the falling rhythm of music or speech) and 'cascade' (water falling in steps). Through Old French 'cheoir' it gave us 'chance' — because what 'falls out' is what happens by luck. Medieval Latin 'casus' (a falling, a case) passed through Old French 'cas' into English as 'case'. Even 'accident' traces back here: Latin 'accidere' (to fall toward, to happen) → Latin 'accidens' → Old French 'accident' → English 'accident'.
The specific verb 'coincidere' — falling together — appears to have been a creation of Medieval Latin scholars, used in philosophical and mathematical writing to describe two lines, points, or arguments that landed on exactly the same position. It was a technical term before it was an everyday one, used to describe geometric figures that perfectly overlapped or logical propositions that perfectly agreed.
The word entered English in the early 18th century, around the 1700s, primarily through the language of science and philosophy. Early English users borrowed it almost directly from Medieval Latin or from its French cousin 'coïncider', which had developed along a parallel track. At first, English writers used 'coincide' mainly in abstract or intellectual contexts — two theories might coincide, or two geometric lines. The sense of two events happening at the same time grew naturally from this, since events that 'fall together' in time occupy the same moment just as two points can occupy the same position in space.
What is perhaps most surprising about 'coincide' is how its everyday use has drifted from its original precision. In mathematics and philosophy, coincidence was an exact, total overlap — two things being perfectly identical in position or meaning. In modern English, we use 'coincide' more loosely, often to mean a rough overlap or an approximate agreement. And its close relative, the noun 'coincidence', has taken on an almost magical flavour — something surprising, unlikely, and tinged with mystery. Yet the root is purely mechanical: things simply fell in the same direction and landed together. The wonder, it turns out, was added by us.