The transfer of wealth from India to England for which Indian got no proportionate economic return, is called the Drain of Wealth.
Till the Battle of Plassey, the European traders used to bring gold into India to buy Indian cotton and silk. However, after the conquest of Bengal, the British stopped getting gold into India. They began to purchase raw material for their industries in England from the surplus revenues of Bengal. Thus, began the process of plundering India's raw materials, resources and wealth to England.