/ ˈwɪndfɔːl; NAmE / noun
1.
an amount of money that sb/sth wins or receives unexpectedly :
The hospital got a sudden windfall of £300 000.
windfall profits
The government imposed a windfall tax (= a tax on profits to be paid once only, not every year) on some industries.
2.
a fruit, especially an apple, that the wind has blown down from a tree