Making what could be called an appeal to purity as a way to dismiss relevant criticisms or flaws of an argument.
This fallacy is a form of circular reasoning, in that it attempts to include a conclusion about something in the very definition of the word itself. It is therefore also a semantic argument.
The term comes from the following example:
If Ian claims that all Scotsman are brave, and you provide a counter example of a Scotsman who is clearly a coward, Ian might respond, “Well, then, he’s not a true Scotsman.”
In essence Ian claims that all Scotsman are brave by including bravery in the definition of what it is to be a Scotsman. This argument only establishes that Ian has a different definition of a commonly used word and provides no meaningful addition to the discussion.
This is used as a means to derail the argument at hand into one of semantics, in the hopes of avoiding concession.