What is the butterfly effect as defined in the book, Isaac’s Storm: A Man, a Time, and the Deadliest Hurricane in History?

Ernest Zebrowski, Jr., looks at the evolution of storms, studying them in reverse. He wonders if something as minute and arbitrary, like a butterfly flitting in one direction rather than another, set off a chain of events. He is fascinated that a storm could start with a drop of rain in one part of the globe and turn into a hurricane in another part of the world.