Are debt certificates that are purchased by an investor.

The answer is Bonds

Bonds are debt certificates purchased by investors. Bonds are usually fixed income investments since they either:

pay annual or semiannual coupons don't pay coupons but are sold at a deep discount

The investor knows that he or she should receive a coupon payment or collect the face value of the bond at maturity.

Bonds are usually issued by government entities or corporations. In case the corporation goes bankrupt, the bond holders have priority in collecting their debt.