Longman Dictionary of Contemporary English
retail price index
noun
EXAMPLES FROM CORPUS
▪ The account pays interest at 4.5% gross over the headline retail price index.
▪ The state pension is increased annually in line with the retail price index.
▪ The value of these stocks is uprated each year in line with changes in the retail price index.
WordNet
retail price index
n. an index of changes in retail prices
Wikipedia
Retail price index
In the United Kingdom, the retail prices index or retail price index (RPI) is a measure of inflation published monthly by the Office for National Statistics. It measures the change in the cost of a representative sample of retail goods and services.
As the RPI was found not to meet international statistical standards, since 2013 the Office for National Statistics no longer classifies it as a "national statistic", emphasizing the consumer price index instead.