A formal debt agreement. It refers to both the agreement and the document that verifies it. It is usually issued by companies and is generally supported by security over some property of the debtor. If the debtor defaults, the creditor can take and sell the property.
Debentures are often transferable, so the creditor can sell it and there are markets on formal stock exchanges that deal in types of debenture. It is sometimes referred to as debenture stock. A mortgage is a type of debenture but one that is always secured, usually against land.