convertible currency
US /kənˌvɜːr.tə.bəl ˈkɜːr.ən.si/
UK /kənˌvɜːr.tə.bəl ˈkɜːr.ən.si/

1.
a currency that can be freely exchanged for other currencies without restrictions
:
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The euro is a widely accepted convertible currency.
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Many developing countries strive to make their currency a convertible currency.