A more thorough explanation:

A periodic-payment-plan certificate is a type of security that provides for a series of periodic payments by the holder. It represents an undivided interest in certain specified securities or in a unit or fund of securities purchased wholly or partly with the proceeds of those payments. The term also includes any security whose issuer is also issuing the certificates described above and whose holder has substantially the same rights and privileges as those holders have upon completing the periodic payments for which the securities provide.

One example of a periodic-payment-plan certificate is a certificate of deposit (CD). A CD is a type of savings account that requires the account holder to deposit a certain amount of money for a fixed period of time. The account holder receives interest on the deposit, and the interest is paid out periodically. At the end of the fixed period, the account holder receives the original deposit plus the accumulated interest.

Another example of a periodic-payment-plan certificate is a bond. A bond is a type of debt security that represents a loan made by an investor to a borrower, typically a corporation or government. The borrower agrees to pay the investor a fixed rate of interest over a set period of time, and at the end of the period, the borrower repays the principal amount of the loan.

These examples illustrate the definition of a periodic-payment-plan certificate because they both involve a series of periodic payments by the holder and represent an interest in a specified security or fund.