No true Scotsman
Etymology
The philosopher Antony Flew illustrated the move in the 1970s with a Scotsman who reads that an Aberdeen man committed a crime and insists "no true Scotsman would do such a thing." Each exception is ruled out by tightening the definition of "true." The result is a claim that can never be proven wrong because it keeps rewriting itself.
noun phrase · /noh TROO SKOTS-man/
Defending a generalization by redefining its terms to exclude any counterexample.
What it names
Catching it stops a claim from being made unfalsifiable through endless "that doesn’t count" edits.
Example
"No real scientist doubts this" — and when shown one who does, he replies she isn’t a real scientist: no true Scotsman.
Related idea
Like special pleading, it invents an ad hoc exception, but specifically to protect a sweeping generalization.