'Money' and 'currency' should not be used interchangeably. Currency is the money of a given state. In the U.S., it is Federal dollar notes in circulation.
Money is what we use every day to pay our liabilities, and this includes currency and, to a much bigger extent, our bank account balances that provide the source for our check, wire transfer, debit card, and credit card payments.
Then, there is another form of 'currency' that exists in the books of the banks, and is equivalent to Federal notes, yet is 'immaterial'. This is the account balance that a bank can hold at the Federal Reserve Bank of its district, called 'bank reserves'.