A slippery slope argument (SSA), in logic, critical thinking, political rhetoric, and caselaw, is an argument in which a party asserts that a relatively small first step leads to a chain of related events culminating in some significant (usually negative) effect. The core of the slippery slope argument is that a specific decision under debate is likely to result in unintended consequences. The strength of such an argument depends on whether the small step really is likely to lead to the effect. This is quantified in terms of what is known as the warrant (in this case, a demonstration of the process that leads to the significant effect). This type of argument is sometimes used as a form of fearmongering in which the probable consequences of a given action are exaggerated in an attempt to scare the audience. However, differentiation is necessary, since, in other cases, it might be demonstrable that the small step is likely to lead to an effect.

The fallacious sense of "slippery slope" is often used synonymously with continuum fallacy, in that it ignores the possibility of middle ground and assumes a discrete transition from category A to category B. In this sense, it constitutes an informal fallacy. In a non-fallacious sense, including use as a legal principle, a middle-ground possibility is acknowledged, and reasoning is provided for the likelihood of the predicted outcome. Other idioms for the slippery slope argument are the thin end/edge of the wedge, the camel's nose in the tent, or If You Give a Mouse a Cookie.