Currency can mean two things:
- a unit of value, like Euros or ounces of gold
- obligations issued from a particular source, like US Federal Reserve Notes
For regular single-issuer currencies, currency is not ambiguous because there is only one issuer and one unit of value. In Ripple, however, there are many different issuers and many units of value for the obligations issued, and no one-to-one correspondence between them in the system.
In Ripple it is usually more clear to use unit of value or obligations, depending on which aspect you are referring to.