The correct answer is "exchange rate."
Each currency has a changing value relative to other currencies. This is referred to as the currency's exchange rate.
The exchange rate is a term in finances that means that every currency has value. When this currency needs to be changed in other currency, you have to take into consideration the difference in the value of both to establish the right amount to be changed. For instance, if you want to change two American dollars to Mexican pesos, the actual exchange rate is 18 pesos per dollar. So you do the numbers and for the two dollars, you will receive 36 pesos.