The term "debenture," as used in modern legal documents, refers to a medium- to long-term debt security issued by a corporation that is not secured by collateral. In more common financial parlance, a debenture would be referred to as a corporate bond, and most corporate bonds are in fact debentures. However, legal writers often reserve the term "bond" for debt securities that are secured by collateral, or for government (as opposed to cporate) debt securities. For more details on the evolution of this terminology, see Commentaries on Model Debenture Indenture Provisions, American Bar Foundation 1965, pp. 7-8.
The legal terms of the debenture are typically set forth in a document called an indenture. In fact, the existence of an indenture and indenture trustee is what typically distinguishes a debenture from a note, which is a more general term for debt that may not be issued under an indenture. The indenture is a contract between the corporate issuer and trustee that acts on behalf of the holders of the debentures.
Debentures that are offered to the public and subject to the Securities Act of 1933 are subject to the provisions of the Trust Indenture Act of 1939, which requires that the debentures be issued pursuant to an indenture with a trustee.