Capital gains are the profits you earn when you sell a capital asset (like stocks, bonds, real estate, or a business interest) for more than you paid for it.
Capital gain = sale price − your cost basis (generally what you paid, plus/minus certain adjustments).
Often categorized as short-term (held 1 year or less) vs. long-term (held more than 1 year), with different tax treatment in many jurisdictions (including the U.S.).
