http://en.wikipedia.org/wiki/Consumer_price_index

A consumer price index (CPI) is a measure estimating the average price of consumer goods and services purchased by households. A consumer price index measures a price change for a constant market basket of goods and services from one period to the next within the same area (city, region, or nation).[1] It is a price index determined by measuring the price of a standard group of goods meant to represent the typical market basket of a typical urban consumer.[2] Related, but different, terms are the United Kingdom's CPI, RPI, and RPIX. It is one of several price indices calculated by most national statistical agencies. The percent change in the CPI is a measure estimating inflation. The CPI can be used to index (i.e., adjust for the effect of inflation on the real value of money: the medium of exchange) wages, salaries, pensions, and regulated or contracted prices. The CPI is, along with the population census and the National Income and Product Accounts, one of the most closely watched national economic statistics.