A Slippery Slope argument (also known as absurd extrapolation, thin edge of the wedge, camel's nose or domino fallacy) is one that warns of the potential harmful consequences of an action and argues that they are inevitable given the proposed course of action. It is not necessarily fallacious, it depends on whether the harmful outcomes are indeed inevitable. Where it is a fallacy is where it ignores the possibility of middle ground.
Generally there are two types of slippery slope argument. The first is the 'causal' version which posits that an action will cause a secondary action, which will cause a third and so on until the detrimental action inevitably happens. The second is a 'judgmental' slippery slope, when a subject decides on action A, they will rationally have to take detrimental action B and or C in order to remain logically consistent.
Examples are often used in politics: Arizona senator Jeff Flake argued on National Public Radio that if the U.S. is going to bar people on a federal terrorist watch list from buying guns, “that argument could go further and say just put every American on the list.” Or as former Conservative Chairman Lord Tebbit said about same-sex marriage: "It's like one of my colleagues said: we've got to make these same sex marriages available to all. It would lift my worries about inheritance tax because maybe I'd be allowed to marry my son. Why not? Why shouldn't a mother marry her daughter? Why shouldn't two elderly sisters living together marry each other?"