to correct the OP
currency is anything used as a medium of exchange
That's not correct!
Currency is not ANYTHING used as a medium of exchange, you can use many things as a medium of exchange which I'm sure is obvious around you. You can purchase your shoe and other products and properties with gold or other products and properties, which makes your definition faulty.
Currency is rather a LEGAL TENDER (fiat) note and others backed by the central bank of a country.
If currency is not anything used as medium of exchange, so tell me what is your definition of currency? Their are so many features of currency which one of them is as medium of exchange which bitcoin has this feature and other better feature than other currencies. If you say a currency must be backed by government that means you are trying to say bitcoin is not a currency, so far as bitcoin is solving the problem of people and its accepted by many these makes it a very good currency. The quality bitcoin has as made it better than the currency back by the government.