A chain reaction was a series of events, such as chemical reactions, where the product of one reaction would become a reactant for the next, and so on.
In Julian Bashir, Secret Agent, one of the villains, Hippocrates Noah, had gathered the world's leading seismologists and constructed a series of lasers designed to cause a chain reaction that would flood Earth via a series of extremely violent earthquakes. (DS9: "Our Man Bashir")