The butterfly effect was a theory that stated that a small cause could create a greater effect over time.
In 2259, Brad Boimler worried that his having made Spock laugh had caused the butterfly effect, and that it would affect the course of time, keeping the Vulcan from assuming his place in history. Christine Chapel assured him that Spock was in a good mood before Boimler travled back in time. (SNW: "Those Old Scientists")