The "No True Scotsman" fallacy  is a logical fallacy where one  

changes the definition of a term to exclude  a counterexample and protect their assertion.

For instance, if someone claims "No  Scotsman puts sugar in his porridge,"  

and is countered with "But my uncle from  Scotland does," they might respond "Well,  

no true Scotsman puts sugar in his porridge."

The fallacy involves subtly shifting  the criteria of 'being a Scotsman'  

to fit the desired conclusion,  ignoring any valid exceptions.

This fallacy is often used to dismiss  evidence that could disprove a person's  

belief or argument, making it a form of circular  

reasoning. It's named after a hypothetical  conversation about the habits of Scotsmen.