Money is a resource with a set of characteristics that are embodied in different combinations of monetary assets (forms of money).
The main characteristics of money are: trustworthiness, divisibility, maintenance of value over location & time, limitation of supply, and convenience. These characteristics allow money to be used in accounting and recording of transactions and wealth, as well as allowing forms of money to be used as mediums of exchange and savings. Monetary assets include notes and coin, bank loans/deposits, credit and debit cards, and various government-backed and short-term bills of exchange.
Since commercial banks can generate loans if consumers or firms request them, the supply on money is endogenous to the monetary system.
“Endogenous money on 21st Century Keynesian economics” – Barker, 2010
Barker, T. (2010) “Endogenous money in 21st Century Keynesian economics”, in Arestis, P. and Sawyer, M. (eds), 21 Century Keynesian Economics, Palgrave Macmillan. Available on Academia.
Fontana, G. (2009) Money, Uncertainty and Time, Routledge. DOI:https://doi.org/10.4324/9780203503294.
Skidelski, R. (2018) Money and Government: A Challenge to Mainstream Economics, Allen Lane, London. Available on Google Books.
Simmel, G. (1900) The Philosophy of Money. Description available on Wikipedia.