By the definition, currency is standardization of money in any form; and money is any item or record that is generally accepted as payment. Therefore currency could be anything you can use as a medium of exchange, if both end agreed to pay/exchange a cow for a chicken then it is a currency. I think this is a general misconception about the currency and money, I even confused these two at first honestly.

You are talking about country currency, well this one is regulated by the country's government to host and have an organize exchange and standardization. I can buy your car for my laptop as my currency, if you want to. Basically that's how currency works.