A
coin of account is a unit of money that does not exist as an actual coin (that is, a metal disk) but is used in figuring prices or other amounts of money. The "mil" was a coin of account in the
United States well into the 21st century: It was equal to one-tenth of a penny, and so to one-thousandth of a dollar (= $0.001), whence the name, which means "thousandth," but there was never such a coin minted, and it is used for figuring taxes, usually either property taxes or sales taxes.