Moving the goalposts
Moving the goalposts is a rhetorical deception in which the qualifications for determining the outcome of a debate are modified after the fact so that an argument which would have otherwise won the debate now no longer meets the minimum requirement to establish victory.
In common usage, this is often done repeatedly throughout a discussion -- i.e. every time a seemingly-conclusive argument is raised, the counter-arguer redefines the terms of the discussion in order to favor their preferred outcome.
The best way to prevent moving the goalposts is to clearly define the terms of the debate before starting. The most rigorous way to do this is to make a statement and then define the debate as being over the truth or falsehood of that proposition -- exactly as written, and nothing else.
If the proposing side loses the debate and then feels that their viewpoint was not accurately represented by the proposition, they can always start over with a new proposition -- but they will no longer be able to claim that their initial proposition is true, which should save time in future discussions.
See Issuepedia:Arguing for further debate guidelines.