Inflation is a rise in general level of prices of goos and services over time. Although "inflation" is sometimes used to refer to a rise in the prices of a specific set of goods or services, a rise in prices of one set (such as food) without a rise in others (such as wages) is not included in the original meaning of the word. Inflation can be thought of as a decrease in the value of the unit of currency. It is measured as the percentage rate of change of a price index but it is not uniquely defined because there are various price indices that can be used.