occasional income; temporary income
Primarily used in Japanese tax law to refer to income that is not regular, such as winnings, prizes, or one-time gains. In everyday conversation, it may be used loosely for any irregular earnings.
Prize money won in a contest is taxed as occasional income.
副業の収入が一時所得にあたるかどうか税理士に相談した。
I consulted a tax accountant about whether my side-job income qualifies as occasional income.