Windfall profit or Windfall Gains is an unexpected profit.

For instance:

winning a lottery

unforeseen inheritance

Shortage of Supply or Sudden Increase in Demand

The main feature of the Windfall Profit is it is temporary in nature. In India, a Tax on Windfall Profit is levied depending on the various parameters. This Tax is called Windfall Tax. Recently, This Tax was in the news after the Indian Government levied a Windfall tax on fuel exports.