Banknote

Gone are the days of lugging around goods to barter, or heavy sacks of gold or silver. Now we can use banknotes (commonly called paper money, bills, notes, bucks, clams, dollars, etc.) to conduct transactions on a far more civilized level.

Technically a central bank's "IOU", it is a promissory note to pay the amount stated on it to the bearer on demand. While it's only a note (since it's a renewable resource and therefore not limited by the scarcity required to assign inherent value), it is considered to be legal tender and, along with coins, is the bearer form (security which can be traded without record of ownership) of modern currency.