Final answer:
Reserved powers help define federalism in the United States by giving states the flexibility to govern themselves.
Step-by-step explanation:
Reserved powers help define the nature of federalism in the United States by ensuring flexibility for the states to govern themselves. The Tenth Amendment of the Constitution establishes reserved powers, stating that any powers not delegated to the United States or prohibited to the states are reserved for the states or the people.
This means that if the Constitution does not specifically grant a power to the federal government and does not restrict the state government from exercising that power, then the state has the authority to control and govern that area.