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?
These criteria are reasonable and logical, although bitcoin doesn't meet all, I think it can still be considered as a currency since it can buy something. And even though it isn't considered and accepted by everybody today, we cryptonians don't mind at all since we're making a fortune out of it.