The Butterfly Effect is the notion that small changes can cause larger ultimate changes with no easily apparent logical pattern. The breeze from a butterfly flapping its wings could ultimately cause or prevent a hurricane.
The phrase entered alternate history literature through Ray Bradbury's short story "A Sound of Thunder" (1952)[1], where a time traveler accidentally kills a butterfly in the Cretaceous Period, causing massive social changes to the United States on his return.
Any changes in an alternate history which are possible but not necessary, such as Roanoke, Virginia keeping its earlier name of "Big Lick" in Harry Turtledove's Southern Victory (or TL-191) novels, or 18th-century European military doctrine in the Atlantis Series being slightly different than in OTL, are considered to be the result of the butterfly effect.
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- ↑ Included in The Best Time Travel Stories of the Twentieth Century.