гос. фин., брит. Windfall tax
Definition (UK specific)
A one off tax imposed by the government in the budget of July 1997 on the profits of privatised utilities companies. It was applied to fund the government's Welfare to Work programme which was instigated to provide employment and to help lone parents and the disabled for the period up to 2002. The profits were assessed as the difference between a company's privatised sale price and its average value over the subsequent four years. The tax was levied on a number of companies including BAA, BG, British Telecom, British Energy, Centrica, National Power, PowerGen, the regional electricity companies and the water and sewerage companies and payable in two instalments on or before 1st December 1997 and 1st December 1998.
похоже есть еще одно значение = windfall profits tax?
Англо-русский экономический словарь.