Convertible debentures are a type of financial instrument issued by companies, which can be converted into shares at a later time. These debentures are known as "convertible" because they can be converted into shares based on certain conditions. Fully Convertible Debentures are those which can be entirely converted into equity shares of the company, while partially convertible debentures only allow for a portion of the value to be converted. This allows investors to have the option to convert their investment into shares, potentially increasing their returns.