There is a long history of disagreements on the definition of money. Having to distinguish two fundamental aspects of the problem has given rise to these controversies. Firstly, it has to be asked: What are the qualities that one thing must have in order to be called money? The answer to this question has focused fundamentally on it being a means of payment, a store of value and a unit of account. Secondly, it has to be asked: What are the particular things that refer to money at a specific moment? The answer to this question concerns the empirical content of the concept of money. It is especially relevant to be able to measure the amount of money that exists in circulation and establish specific monetary measures.