What is Foreign-exchange Reserves?

Legal Definition
Foreign-exchange reserves (also called forex reserves or FX reserves) is money or other assets held by a central bank or other monetary authority so that it can pay if need be its liabilities, such as the currency issued by the central bank, as well as the various bank reserves deposited with the central bank by the government and other financial institutions. Reserves are held in one or more reserve currency, mostly the United States dollar and to a lesser extent the EU's euro, the British pound sterling, and the Japanese yen.
-- Wikipedia
Legal Definition
The countries gold holdings and foreign currency in banks. It includes special drawing rights and exchange reserve balances with the IMF.