As the theories became more carefully specified in the 19th and early 20th centuries, they included a variable called the money supply.
Money is often defined in terms of the three functions or services: as a medium of exchange, as a store of value and as a unit of account.
Without money, all transactions would have to be conducted by barter, which involves direct exchange of one good or service for another.
Money also functions as a unit of account, providing a common measure of the value of goods and services being exchanged.
Short fixed-term deposits (such as thirty-day treasury bills) and government bonds which are close to their maturity date are examples of assets which are not quite as liquid as a bank account that permits immediate withdrawal, but in many circumstances the difference is not important.