Butterfly effect

The butterfly effect, used to describe many chaotic phenomena, was first described as such in reference to weather: that the beating of a butterfly's wings in Brazil might set off a tornado in Texas months later¹. Chaotic systems such as weather are said to be sensitively dependent on initial conditions, in that some small change may trigger a slightly larger change, which then triggers a slightly larger one, and so on, until it becomes completely impossible to predict long-range effects.

See Chaos theory.

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¹ Edward Lorenz, in a paper in 1963 given to the New York Academy of Sciences, said: "One meteorologist remarked that if the theory were correct, one flap of a seagull's wings would be enough to alter the course of the weather forever." Later speeches and papers by Lorenz used the more poetic butterfly.