Originally from the British phrase where a goal post in a foot ball based sport is moved to advantage one side and disadvantage the other side, this logical fallacy denotes the situation where in the process of making an argument, the goal is shifted to make that argument either easier or harder. The fallacy resides in the moving of the goal whilst the process is taking place, rather than defining the goal first, then following the arguments to reach the target – either to support it or discredit it.
You may recall in the animated movie Robin Hood, by Walt Disney Productions, where Robin Hood (the fox) is disguised as a bird and attempting to shoot some targets in a competition to win a kiss from Maid Marianne (if not, go back and watch the movie – it’s awesome). His opponent, the sherrif of Nottingham, shoots and the target jumps up and gets in the way of the arrow, making a bulls eye. This shifted goal post allowed an less competent archer to get a successful conclusion. In this example, the person moving the target is incapacitated so that Robin can shoot with skill alone, however if the person had not been incapacitated, they could have shifted the goal out of the way of the successful shot, creating a miss.
As a logical fallacy, the target can be moved closer for a poorly constructed experiment or argument to succeed where it shouldn’t (such as for the sherrif of Nottingham), or the target can be moved further away, to make a normally successful experiment or argument fail. This tactic is often used by proponents of psuedoscience, allowing their science to seem legitimate by using close goal posts, that is poor criteria, but then creating overly strict and stringent criteria for legitimate science to make it look faulty. Properly applied science uses the same criteria for all rather than shifting the rules.
An example from the “Supplement Industry” of this is defining their product as a diet supplement rather than a medicine, even though they make very medicine like claims [http://archinte.jamanetwork.com/article.aspx?articleid=647749]. The advertising targets legitimate medication as having nasty side effects and being full of chemicals, while the supplement itself does not have the advertised ingredient (1/3 in the USA), contain contaminants (1/5 in the USA) and no one really knows what the side effects are of the supplement, because there is no requirement to test them (according to the FDA in the USA) [http://theness.com/neurologicablog/index.php/whats-in-your-herbal-supplement]. By comparing the two as if they followed the same criteria yet the goal posts are shifted because medication must meet certain criteria such as testing, field trials, tracking, random checks etc, while supplements require none so can claim whatever they like without the requirement or burden of proof.