Moving or shifting the goalposts is named after the idea of moving the goalposts during a football game so as to gain an advantage over the opposing team. As a logical fallacy, it is committed when one party to a dispute arbitrarily changes the standards or criteria that their argument (or their opponent’s argument) must meet in order to be deemed successful. Most commonly this involves wither establishing a lower standard for one’s own argument, or a higher standard for the opponent’s argument, or both. There is, of course, nothing wrong in general with varying one’s standards in accordance with the topic under discussion, however doing so selectively to advantage one’s own position involves committing the fallacy of shifting the goalposts. Often the fallacy is committed when one side to a dispute runs out of rebuttals or arguments to give, and instead switches to either increasing or lowering (depending upon whether they are attacking or defending an argument) the standards for what constitutes a successful argument or sufficient evidence.
This fallacy is often associated with statements such as ‘show me one piece of evidence that…’, or ‘give me one good reason why…’, or similar. Very often when some evidence is provided or a reason given, the person making this statement will not alter their opinion, but will then demand still more evidence and additional reasons. Requiring more evidence or reasons is not in itself fallacious, but to do so after having explicitly or implicitly claimed that only a single argument or piece of evidence would be sufficient is to apply inconsistent epistemic standards, which is fallacious. Arbitrarily changing the standards in order to suit one’s argument is disingenuous and characteristic of sloppy reasoning.
Moving the goalposts: overview from Exposing PseudoAstronomy
Moving the goalposts: overview from Logically Fallacious