Motte-and-bailey fallacy
The motte-and-bailey fallacy (named after the motte-and-bailey castle), also called the castle and courtyard, is a form of argument and an informal fallacy where an arguer conflates two positions that share similarities: one modest and easy to defend (the "motte") and one much more controversial and harder to defend (the "bailey"). The arguer advances the controversial position, but when challenged, insists that only the more modest position is being advanced.