What is an allusion in A Sound of Thunder?

The allusion in A Sound of Thunder is to the butterfly effect. The butterfly effect is a concept that states that small changes in a system can have large effects over time. The name comes from the idea that the flapping of a butterfly's wings can eventually cause a tornado. In A Sound of Thunder, the protagonist, Eckels, accidentally steps on a butterfly while on a hunting expedition. This small change eventually leads to a large change, as it alters the timeline and Eckels finds himself in a world that is very different from the one he left. The allusion to the butterfly effect suggests that even small actions can have unintended and far-reaching consequences.