Money earned outside a regular paycheck: freelancing, gig apps, a small business, selling things. It arrives with no tax withheld, and you owe both income tax and self-employment tax (15.3%, which is both halves of Social Security and Medicare that an employer would otherwise split with you). The IRS expects it paid through the year in four estimated payments, due around April 15, June 15, September 15, and January 15.