The Impossible Trinity (also known as the Inconsistent Trinity, Triangle of Impossibility or Unholy Trinity) is the hypothesis in international economics that it is impossible to have all three of the following at the same time:
The point is that you can't have it all: A country must pick two out of three. It can fix its exchange rate without emasculating its central bank, but only by maintaining controls on capital flows (like China today); it can leave capital movement free but retain monetary autonomy, but only by letting the exchange rate fluctuate (like Britain--or Canada); or it can choose to leave capital free and stabilize the currency, but only by abandoning any ability to adjust interest rates to fight inflation or recession (like Argentina today, or for that matter most of Europe).
In the modern world, given the growth of trade in goods and services, capital controls are easily evaded. In addition, capital controls introduce numerous distortions. Hence, there is virtually no important country which has an effective system of capital control. Under these conditions, the Impossible Trinity asserts that a country has to choose between reducing currency volatility and running a stabilising monetary policy: it cannot do both.