The Principles of '98 refer to the American political position after 1798 that individual states could both judge the constitutionality of federal laws and decrees and refuse to enforce those that were deemed unconstitutional. That refusal is generally referred to as "nullification" but has also been expressed as "interposition:" the states' right to "interpose" between the federal government and the people of the state.
The Principles of '98 were widely promoted in Jeffersonian democracy, especially by the Quids, such as John Randolph of Roanoke, but never became law.