A windfall gain (or windfall profit) is any type of income that is unexpected.

Windfall Gains and Stock Market Participation
The psychology of windfall gains.
Windfall Gains, Political Economy and Economic Development
The Effect of Windfall Gains on the Sunk-Cost Effect
Self-Employment and Windfall Gains: Evidence from the Swedish Lottery
Self-Employment and Windfall Gains in Britain: Evidence from Panel Data
House Price Shocks, Windfall Gains and Hours of Work: British Evidence *
Entrepreneurship, windfall gains and financial constraints: Evidence from Germany ☆
Precautionary principle as a rule of choice with optimism on windfall gains and pessimism on catastrophic losses
Social sharing and risk reduction: Exploring a computational algorithm for the psychology of windfall gains