Fixed exchange rate is the rate which is officially fixed by the government or monetary authority and not determined by market forces. Only a very small deviation from this fixed value is possible. In this system, foreign central banks stand ready to buy and sell their currencies at a fixed price. A typical kind of this system was used under Gold Standard System in which each country committed itself to convert freely its currency into gold at a fixed price.
In other words, value of each currency was defined in terms of gold and, therefore, exchange rate was fixed according to the gold value of currencies that have to be exchanged. This was called mint par value of exchange. Later on Fixed Exchange Rate System prevailed in the world under an agreement reached in July 1994.