Federalism is:

A. the doctrine that says the Constitution is the supreme law of the land
B. the sharing of power between a federal government and the states
C. the belief that the states should not have to the power to make their own laws
D. the power of the president to declare national emergencies

Federalism is a model of government where states are fully recognized yet they share power with the Union and state share some degree of sovereignty pursuing higher goals.

In the US national and state governments are interdependent, and the Federalists advocate for a strong presence of the central government in attaining common goals that benefit both the states and the Union.

The Constitution underlines to which extent the Union can act, and what degree of sovereignty is to be demanded by local actors and states.

Examples of other federalist models of government:

Argentina - With 23 provinces and Buenos Aires is an autonomous city.
Australia - Australia became the Federation of Australia which established the Commonwealth of Australia.
Austria - 9 states and is a semi-presidential, parliamentary representative democracy.