The tax on the profit when you sell something for more than you paid: stocks, funds, crypto, property. Held a year or less, the gain is 'short-term' and taxed like wages. Held more than a year, it's 'long-term' and taxed at 0%, 15%, or 20% depending on income. For 2026 the 0% rate covers taxable income up to $49,450 single or $98,900 married filing jointly. Losses offset gains. None of this applies inside a 401(k), IRA, or HSA.