The state's right to resist federal laws that the state considers unconstitutional.

the action of interjecting or interposing an action or remark that interrupts

The idea that a state, having retained its sovereignty in a federal system, can interpose its authority between the central government and an individual, to protect its citizens from illegal or unconstitutional action. (See the discussion of nullification in the text and states' rights below.)

Interposition, in the context of the Kentucky and Virginia Resolutions, refers to an asserted right of U.S. states to protect their individual interests from federal violation or any abridgement of states' rights deemed by those states to be dangerous or unconstitutional.