RPI is an abbreviation of Retail Prices Index, a measure of inflation in the UK. It is based on the change in value of the arithmetic mean of a notional basket of goods and services bought by the average family in the UK (which is adjusted over time to reflect changes in people’s spending patterns). It used to be the main official measure of UK inflation but has been supplanted by the CPI or Consumer prices Index.
Variations of RPI include RPIX (which excludes mortgage payments), RPIY (which is RPIX excluding taxes like VAT and Council Tax) and RPIJ (which is RPI calculated on a geometric rather than an arithmetic basis).