The purpose of money - a medium to make exchange goods and services more efficient than barter.

That doesn't change the underlying principle of barter - which is, "I value what you have more than what I have, and you value what I have more than what you have, so we swap". In other words, it's about exchange of value. Which leads to the definition, that:

Money is a system for communicating about value.

I would even say that money is a "language" for communicating about value. And value is subjective, please keep that in mind!

Read as blog post:
http://ellehauge.net/what-is-money/