In economics, a nominal value is an economic value expressed in historical nominal monetary terms. By contrast, a real value is a value that has been adjusted from a nominal value to remove the effects of general price level changes over time and is thus measured in terms of the general price level in some reference year (the base year). For example, changes in the nominal value of some commodity bundle over time can happen because of a change in the quantities in the bundle or their associated prices, whereas changes in real values reflect only changes in quantities. The process of converting from nominal to real terms is known as inflation adjustment. Real values are a measure of purchasing power net of any price changes over time. For example, nominal income is often restated as real income, thus removing that part of income changes that merely reflect inflation (a general increase in prices). Similarly, for aggregate measures of output, such as gross domestic product (GDP), the nominal amount reflects production quantities and prices in that time period, whereas the differences between real amounts in different time periods reflect only changes in quantities. A series of real values over time, such as for real GDP, measures quantities over time expressed in prices of one year, called the base year (or more generally the base period). Real values in different years then express values of the bundles as if prices had been constant for all the years, with any differences due to differences in underlying quantities. The nominal/real value distinction can apply not only to time-series data, as above, but also to cross-section data varying by region. For example, the total sales value of a particular good produced in a particular region of a country is influenced by both the physical amount sold and the selling price, which may be different from that of the country as a whole; for purposes of comparing the economic activity of different regions, the nominal output of the good in that region can be adjusted into real terms by repricing the goods at national-average prices.