hmm, definition i don't know right now but i can point to its origin.
in the old days, a goods or services barter system was in place, where a party had something in surplus, or wanted something for immediate/future use, would go to another party and offer what goods or services he had and would in turn get what he desired... .. the desire was to have get something by offering something because the society was not scientific therefore "get for free" was not thought.
this primitive barter system was direct but was inefficient... maybe what he had was not required by the other party or maybe the other party desired more.
this necessiated the introduction of more negotiable items whose value was known widely... these items were precious stones, gold, silver, jewellery, others precious metals.
this system functioned well when a society was a village or a town and a citizen went to other lands as well.
but when kingdoms started and made way for empires, it was thought that a newer form of agent-item would make exchange of goods and items within a realm easier and more efficient... this became the money system, originally made in precious metal but in china introduced as a paper item.
it was also wrongly thought that this agent-item ( money ) when deposited in a centralized place and under control of the ruler would help compare the importance of a kingdom or empire with other kingdoms and empires... i speak of the monetary wealth of a king or emperor in the treasury which need not translate into comforts for the citizens there.
you asked for a definition which i did not have... you can perhaps use the above with your own thoughts and research to arrive upon a definition... i may have missed out some things in the above... maybe "the capital" by karl marx may have a definition... that book is full of mathematical equations so i have not read it.