What is described as a state's refusal to recognize an act of Congress that it considers unconstitutional?

The term that describes a state's refusal to recognize an act of Congress that it considers unconstitutional is nullification. This concept was prevalent in the early 19th century, particularly associated with the Southern states, which argued that they had the right to invalidate federal laws that they believed violated states' rights or the Constitution. The debate over nullification was notably tied to the issues of states' rights and the balance of power between state and federal governments, which became prominent during conflicts like the Nullification Crisis in the 1830s.

In contrast, impeachment refers to the process by which a federal official can be charged with misconduct; separation of powers concerns the division of government responsibilities into distinct branches to prevent any one branch from wielding too much power; and judicial review is the power of courts to evaluate the constitutionality of legislative acts and executive actions. These concepts are related to governance and constitutional interpretation but do not match the definition involving a state's rejection of federal law.