A deflator is a value that allows data to be measured over time in terms of some base period, usually through a price index, in order to distinguish between changes in the money value of a gross national product (GNP) that come from a change in prices, and changes from a change in physical output.

•   It is the measure of the price level for some quantity.

•   A deflator serves as a price index in which the effects of inflation are nulled.

•   It is the difference between real and nominal GDP.

•   It is generally used to convert dollars purchasing power into "inflation-adjusted" purchasing power, thus enabling the comparison of prices while accounting for inflation in various time periods.