Economic Union (EU) is the complete harmonization of seperate markets. It is often considered the final stage of economic integration.
In an EU, there is no difference in market policy between members.
For instance, imagine there are three nations: A, B, and C. A and B are in the EU, while C is not. For all regulatory purposes, A is identical to B. Essentially, a new country is created, AB, which subsumes both A and B. Economic Union is a great economic boost to the world, but it realigns the political situation.
Differences between member states are cultural, not legal. If the economic union holds, memories of its constituent states will be lost.
Japan is the most successful economic union in history (see what I mean about the previous regime being forgotten?). Arguments can be made for the European Union and the United States of America, but these countries' federal systems make them more similar to common markets. | 
