No True Scotsman is an intentional logical fallacy which involves the act of setting up standards for a particular scenario, then redefining those same standards in order to exclude a particular outcome.
The Trope Namer and prime example of this sort of behavior is a hypothetical scenario (first told by British philosopher Antony Flew in his 1975 book Thinking About Thinking) in which a Scotsman [...] [tvtropes.org]