From the Point of view of Tenure, Debentures can be:

NCERT (2026-27 edition):
From the Point of view of Tenure – “Redeemable Debentures: Redeemable debentures are those which are payable on the expiry of the specific period either in lump sum or in Instalments during the life time of the company. Debentures can be redeemed either at par or at premium. Irredeemable Debentures: Irredeemable debentures are also known as Perpetual Debentures because the company does not give any undertaking for the repayment of money borrowed by issuing such debentures.”