Specific functions (mostly micro-economic)
 

• Unit of account (abstract)
• Common measure of value (abstract)
• Medium of exchange (concrete)
• Means of payment (concrete)
• Standard for deferred payments (abstract)
• Store of value (concrete)

General functions (mostly macro-economic and abstract)

• Liquid asset
• Framework of the market allocative system (prices)
• A causative factor in the economy
• Controller of the economy

The table above comes from page 27 of A History of Money.

Not everything used as money as all the functions listed above. Furthermore the functions of any particular form of money may change over time. As Glyn Davies points out on page 28:

"What is now the prime or main function in a particular community or country may not have been the first or original function in time, while what may well have been a secondary or derived function in one place may have been in some other region the original which gave rise to a related secondary function... The logical listing of functions in the table therefore implies no priority in either time or importance, for those which may be both first and foremost reflect only their particular time and place."

He goes on to conclude from this that the best definition is as follows:

Money is anything that is widely used for making payments and accounting for debts and credits.