Butterfly effect

From Turtledove

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 "The Sound of Thunder." In that story a time traveller accidentally kills a butterfly millions of years ago, causing massive social changes on his return.

Any changes in an alternate history which are possible but not necessary, such as Roanoake, Virginia keeping its earlier name of Big Lick in Harry Turtledove's Southern Victory (or TL-191) novels, are the result of Butterfly effect.