Anything that can pay a debt or purchase property, in any part of a country, is the money of that country.
Every civilized country has its own minted or printed money.
The usual mediums of circulation are gold, silver, nickel and copper, the latter alloyed more or less in the United States with nickel.
Government and bank bills, while having all the purchasing power of gold, are simply promises to pay in gold, or other coin of "redemption", the amounts they represent.
The money of one country cannot legally be made to pay a debt in another country, unless both parties to the payment agree to it.
When gold is exchanged to settle the balances of trade between two countries, it is not reckoned, if coined, at its face value, but at its bullion value.
The word "pecunia" meant in ancient Greece and Rome a flock or herd.
In those days live stock were used as a medium of exchange, or money.
We keep the word and often use it as in "pecuniary" affairs, and when we call a moneyless man, "impecunious."