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?
I think bitcoin fulfills all points, the problem of stable value storage is often debated, but if we look together, the real money that we currently use also has fluctuating value, even experiencing a declining average movement