What is a written promise to pay a specified sum called?

A written promise to pay a specified sum is known as a promissory note. This financial instrument is a formal document in which one party (the issuer or maker) commits to paying a specific amount of money to another party (the payee) at a designated future date or on demand. Promissory notes are legally binding and often include details such as the amount borrowed, interest rate, payment schedule, and other terms relating to the repayment.

In this context, a promissory note stands out because it is specifically designed to represent a borrower's promise to repay a certain amount, distinguishing it from other financial documents. A loan agreement, for example, might include a promissory note as one part of its broader contract outlining all the terms of the loan. A bond is a type of debt security that generally indicates borrowing through a more formal process typically involving multiple investors and often secured by collateral. Lastly, a debt security refers to a broader category of financial instruments used to raise funds, which can include bonds, debentures, and other notes, but does not specifically equate to the individual commitment found in a promissory note. Thus, the correct answer highlights the specific nature of this financial promise.