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?
If that is how currency is being defined, I believe as of now bitcoin can not be consider as one. Being only accepted in just some countries already limits its use for transacting. Though it has a value stored, its stability is the problem. The volatility of bitcoin makes it hard to identify its exact value. When it comes to being a unit of account, I think it is still not being consider as one. Though I believe as time goes by bitcoin will achieve it.