A loan to a government or company that pays interest and returns principal at maturity.
A bond is essentially an IOU: you lend money to a government or corporation, which pays periodic interest and repays the principal on a set date. Bonds are generally less volatile than stocks and provide stability and income, which is why portfolios often add more bonds as retirement nears. Their prices fall when interest rates rise, and vice versa.
A retiree might hold more bonds so the portfolio swings less when they need to withdraw.