The ISP is Portugal's excise tax on fuels — the Tax on Petroleum and Energy Products (Imposto sobre Produtos Petrolíferos e Energéticos) — charged as a unit rate per litre on petrol, diesel and other fuels. The government sets and can temporarily cut those unit rates; a recent decision to reduce the rate for road diesel on the mainland aims to lower pump prices and reduce transport costs for drivers and businesses, though it also lowers tax revenue.