MEDIUM
A currency has two sources of value. One is extrinsic and the other is intrinsic. The extrinsic value of a currency is its purchasing power. It can buy milk and bread. It can be used to pay rent. Without that power, currency is worth nothing.
The intrinsic value of a currency is more relative. It is the ease with which it facilitates exchange compared to other available options. Gold can be traded for milk and bread, but gold is difficult to lug around. Goods and services can be bartered for other goods and services, but only in specific markets.
What makes a currency successful is the relative ease with which it mediates all those exchanges.