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"optimum currency area" (OCA), also known as an optimal currency region (OCR) is  is a term used in economics, refers to a geographical region in which it would maximize economic efficiency to have the entire region share a single currency.

It describes the optimal characteristics for the merger of currencies or the creation of a new currency. The theory is used often to argue whether or not a certain region is ready to become a currency union, one of the final stages in economic integration.

An optimal currency area is often larger than a country. For instance, part of the rationale behind the creation of the euro is that the individual countries of Europe do not each form an optimal currency area, but that Europe as a whole does form an optimal currency area.[1] The creation of the euro is often cited because it provides the most modern and largest-scale case study of the engineering of an optimum currency area, and provides a comparative before-and-after model by which to test the principles of the theory.

In theory, an optimal currency area could also be smaller than a country. Some economists have argued that the United States, for example, has some regions that do not fit into an optimal currency area with the rest of the country.

The theory of the optimal currency area was pioneered by economist Robert Mundell. Credit often goes to Mundell as the originator of the idea, but others point to earlier work done in the area by Abba Lerner.

"asymmetric shock" :

A change in microeconomics conditions that affects differently the different parts of a country, or different countries of a region. This is often mentioned as a source of difficulty for countries that share a common currency, such as the Euro Zone.

Example: If Germany has a positive aggregate demand shock and France has a negative aggregate demand shock, then these two countries are experiencing asymmetric shocks.

Having similar, or symmetric, shocks is one of the criteria of an optimal currency area.

Asymmetric shocks make it difficult for the central bank of a monetary union to conduct monetary policy that is beneficial to each member of the union.