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?
This is a successful currency criteria when fiat currency came in the market. But before fiat became currency, that is not the exact definition of a currency. For example gold and silver, they have different value base on what country or place you are in the past.