Non-convertible debentures are simply regular debentures, cannot be converted into equity shares of the liable company. They are debentures without the convertibility feature attached to them. As a result, they usually carry higher interest rates than their convertible counterparts.
NCDs might be the answer to your quest for the investment instrument that offers high returns with moderate risk while giving you the flexibility of choosing between short and long tenures.
An NCD can be both secured as well as unsecured. For secured debentures, which are backed by assets, in case the issuer is not able to fulfil its obligation, the assets are liquidated to repay the investors holding the debentures.