Motte-and-bailey fallacy

The motte-and-bailey fallacy is a way of arguing where someone uses two different ideas that seem similar but are not the same. One idea (the "motte") is easy to defend and not very controversial. The other (the "bailey") is more controversial and harder to defend. When someone argues for the controversial idea but gets challenged, they switch to defending the less controversial one. This makes it look like their original point is still valid, even though they are now arguing something different.