According to Reference.com, a debenture is a certificate or document acknowledging a loan agreement or company's indebtedness without security or collateral provided to the debenture holder. The holder of the certificate receives a fixed rate of return.
Debentures are most often issued by blue-chip corporations and service companies. Typically, a corporation either pays the debenture on a specific date or makes periodic payments. The term is also used regarding a document granting a creditor money due from the government after fulfilling a condition. In the UK, a debenture is very similar to a bond, and debenture stock is like preferred stock.