Successful currencies meet three criteria:
(1) they are in widespread use as medium of exchange
(2) they serve as unit of account
(3) they are a stable store of value.
Most sovereign currencies fully meet all three criteria.
What about Bitcoin?
1. Medium of exchange. The main criteria for something to function as a medium of exchange are that it be robust and portable, Since BTC are virtual, they obviously fulfill these functions.
2. Unit of account. A unit of account needs to have a stable value.