This is a measure of inflation, defined as the all-items retail prices index excluding rent, mortgage interest payments, council tax and depreciation costs.
The September Rossi index is used to update income-related benefits (e.g. Job Seekers Allowance / Income Support).
The index is named after Hugh Rossi, who was the social security minister responsible for its introduction. Introduction of the index was considered necessary in order to avoid double-counting housing and council tax costs (for which separate benefits are available) within the calculation of more general income-related benefits.