Definition of the Consumer Price Index from the Oxford Advanced Learner's Dictionary

the Consumer Price Index

(American English the Consumer Prices Index) (abbr the CPI)
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 but social security benefits in Britain are increased on the basis of the older Retail Price Index.