Money is any item or verifiable record that is generally accepted as payment for goods and services and repayment of debts in a particular country or socio-economic context. The main functions of money are distinguished as: a medium of exchange; a unit of account; a store of value; and, sometimes, a standard of deferred payment. Any item or verifiable record that fulfills these functions can be considered as money.
Here's McConnell et. al. 18th ed., p. 33:
A rather obvious characteristic of any economic system is the extensive use of money. Money performs several functions, but first and foremost it is a medium of exchange. It makes trade easier... [M]oney... is simply a convenient social invention to facilitate exchanges of goods and services... to serve as money, an item needs to pass only one test: It must be generally acceptable to sellers in exchange for their goods and services. Money is socially defined; whatever society accepts as a medium of exchange is money.I was able to download a full text pdf of McConnell, on 2017-07-23, per the Wikipedia economics page. However, that link was busted last time I checked, on 2017-09-05.