DEF: Money or profit received unexpectedly.
EX:
European nuclear energy companies are making large windfall profits because electricity price increases with the price of oil and gas, but the nuclear company do not use these fossil fuels and are not affected by their costs.
Merriam-Webster Online Dictionary
windfall (noun)1.
something (as a tree or fruit) blown down by the wind
2.
an unexpected, unearned, or sudden gain or advantage
