A domino effect is the cumulative effect produced when one event sets off a series of similar or related events, a form of chain reaction. The term is an analogy to a falling row of dominoes. It typically refers to a linked sequence of events where the time between successive events is relatively short. The term can be used literally (about a series of actual collisions) or metaphorically (about causal linkages within systems such as global finance or politics).

The literal, mechanical domino effect is exploited in Rube Goldberg machines. In chemistry, the principle applies to a domino reaction, in which one chemical reaction sets up the conditions necessary for a subsequent one that soon follows. In the realm of process safety, a domino-effect accident is an initial undesirable event triggering additional ones in related equipment or facilities, leading to a total incident effect more severe than the primary accident alone.

The metaphorical usage implies that an outcome is inevitable or highly likely (as it has already started to happen) – a form of slippery slope argument. When this outcome is actually unlikely (the argument is fallacious), it has also been called the domino fallacy.