Money exists in a few dimensions as an agreed upon means of exchange which can be shells, goats, coins and paper bills. A culture or an individual assigns value to each item or service. Most people believe that money as a resource is scarse. But that is not true, which means that money also has a mental and emotional component. Most people carry a lot of unsupported beliefs about money, which they mostly received from childhood. That means that a person's relationship with money and abundance in genera is not even theirs. It is important to analyze beliefs about money and let go of anything that is disempowering.
Some people carry financial challenges on a karmic level, in which case that person may be giving back karmic debts and is experiencing lack.
The simple definition is: money is a means of exchange. Any connotations to money has been created by the human ego.
What are some of your beliefs about money that you've accepted from family or society? Are they really true for you? What would it feel like to release those misconceptions and create a reality free from this baggage?