Money is a tool to measure price, a medium to buy goods, and a means to preserve wealth. It is a contract between the owner of the property and the market about the right of exchange. In essence, it is an agreement between the owners.
There are still a lot of controversies about the nature of money. There are many kinds of money concepts in economics. At first, it was defined by the function of money, and then it was defined as an economic variable or policy variable.