The notion that a small change in the initial conditions of a system can result in large differences in a later state.

Notes:
Every other time travel story could be creatively interpreted as involving some subjectively small change to the past having some subjectively large effect in the future. For this theme to apply, the motivation must indicate why the change is considered small, and the effect large. Typically, this happens when a character points it out more or less explicitly.

References:
https://en.wikipedia.org/wiki/Butterfly_effect