The price of one currency expressed in terms of another currency is called the:
Exchange rate - the rate at which the currencies are exchanged. This rate can vary frequently, some-times several times per hour, depending on circumstances.
The interest rate is the rate charged on loans - the cost of borrowing.
The base rate is the basic interest rate used for agreeing loans within an economy. This rate is nor-mally set by the government or the central bank. Loans within that economy will usually be at a higher rate than the base rate, often much higher.
The equilibrium rate (more commonly 'equilibrium price') is the rate at which supply and demand meet, not necessarily related to currency.