a list of the prices of some ordinary goods and services which shows how much these prices change each month see also retail price index Culture a measure of the amount by which prices have increased. The CPI is calculated by looking at the average price of a range of things which an average person might buy each month and shows general movements in prices. In the US it is used as the basis for changes in social security benefit payments. In the UK it is the name used for the Harmonized Index of Consumer Prices, the index used in all countries of the EU so that inflation rates of member states can be compared. The CPI is used by the Treasury for the inflation target each year and pensions for public employees are increased on the basis of it. compare retail price index.