The Consumer Price Index (CPI) is a measure of the average change in prices paid by consumers for a basket of goods and services over time. It is calculated by tracking the prices of a representative sample of consumer goods and services, and averaging their price changes for a given period.
The CPI is typically calculated by a government institution that is separate from the Central Bank, like the National Office of Statistics.
Sources:
CPI - Inflation (takes you to an OECD website)
Consumer Price Index (takes you to Wikipedia)
Consumer Price Index (takes you to Investopedia)