Unrelated Business Income Tax (UBIT or UBTI) is a special tax that can apply to retirement accounts like a Solo 401k or IRA if they earn money from certain types of business activities that aren’t related to typical investment income. For example, if your retirement account invests in a business that actively sells products or services—or takes on a loan to buy real estate and earns rental income—some of that income might be considered “unrelated business income” and could be taxed, even though most retirement account earnings are tax-deferred or tax-free. The idea is to prevent retirement accounts from getting an unfair tax advantage when engaging in regular business operations.