The fallacy of definitional retreat occurs when an arguer seeks to escape from a valid counter-argument by redefining a key term without acknowledging that this changes the nature of the debate.

Here is an example:

Pan: "All the Hollywood fantasy movies are really long -- every one of them is over two hours."

Jan: "But The Never-Ending Story is only 94 minutes."

Pan: "That's for kids. I mean adult fantasy movies. They're always two-hours-plus."

Jan: "Well, The Princess Bride is just 98 minutes."

Pan: "That's a comedy. I mean non-comedic adult fantasy movies."

Jan: "What about The Beast Master? It comes in just under two hours."

Pan: "So, I mean non-comedic adult fantasy movies without Tanya Roberts in them."

Pan is exhibiting definitional retreat, narrowing the definition of the "fantasy movies" in question to retain the assertion of them being longer than two hours. It would be better for Pan to revise the original claim instead, saying "many" fantasy movies are longer than two hours, but not all of them.

See also: no true Scotsman.

Image Credit: David Revoy under CC BY 3.0